Means for higher speed inkjet printing

ABSTRACT

A method and apparatus for subdividing and printing regions of a substrate with multiple print heads or multiple print head assemblies which substantially decreases printing time is described. The method includes a feathering method which reduces overlap artifacts which is useful in any printing situation where adjacent regions are printed that could be misaligned, offset, or have slightly different colors.

CROSS REFERENCE

This application claims priority from U.S. Utility application havingSer. No. 11/659,275 filed May 17, 2007 which in turned claims priorityfrom PCT/US2005/028,028 having an International Filing date of Aug. 8,2005 which in turn claims priority from a US Provisional Application ofthe same title having Ser. No. 60/599,395 filed Aug. 6, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Methods and apparatus for subdividing and printing regions of asubstrate with multiple print heads or multiple print head assemblieswhich substantially decreases printing time. The methods include afeathering method which reduces overlap artifacts which is useful in anyprinting situation where adjacent regions are printed that could bemisaligned, offset, or have slightly different colors.

2. Description of Prior Art

Inkjet printing has evolved from early systems with one nozzle which wasscanned across a medium ejecting a droplet of ink when the nozzle ispositioned over a to-be printed area, to systems with multiple nozzlesarrayed on a Nozzleplate in a scanning Printhead (for a single color).Later multiple, different colored Printheads ganged together, andscanned as a unit called a Printhead Array. Also popular is a singleprinthead with a nozzle plate with up to four rows of nozzles, each ofsaid rows ejecting one of cyan, magenta, yellow, or black.

Other implementations of inkjet printing have used Pagewidth Arrayheads, with the nozzle array as wide as the page or substrate to beprinted. In the Pagewidth Array configuration, the Pagewidth PrintheadAssembly would have at least as many nozzles as resolution elements ofeach color to be printed, and might consist of four Pagewidth Arrays,for each of cyan, magenta, yellow and black.

A more complete discussion of inkjet printing and print algorithms canbe found in the Hewlett Packard Journal, February 1994, and available athttp://www.hpl.hp.com/hpjournal/94feb/feb94.htm.

Throughout the history of inkjet printing, designers have struggled tosimultaneously improve throughput, quality, and cost, which generallytrade off against each other.

Many scanning head inkjet printing systems have been designed, withattention to different user needs for size of the substrate, type ofsubstrate, speed, printing fluid, and cost. All scanning head inkjetsystems have in common a basic mechanism of moving the substrate in onedirection, called the paper axis in the case of conventional officeprinters; and moving the nozzles, printhead, or ganged printheads in aperpendicular direction, called the scan axis in the case ofconventional office printers; thus enabling complete coverage of a 2dimensional substrate with print fluid, subject to the interruption ofdroplet ejection by a control mechanism. The terms paper axis refersmore generally to the direction that the substrate is indexed, eventhough the actual substrate might be fabric or other material. The termscan axis refers to the direction that printheads move. The controlmechanism determines the pattern of fluid impinging on the substrate,which pattern generally can be configured to any shape, subject tovarious mechanical, electrical, and fluid flow limitations which canlimit how accurately fluid can be placed, and how fast and accuratelythe scanning head or paper can be repositioned, and the rate at whichfluid can be ejected.

Most scanning head ink jet printers scan a Printhead (or PrintheadArray, composed of multiple printheads) laterally across a page,depositing ink in a Swath Height which is the length of the NozzlePlatesNozzle Array. Then the paper is indexed in the direction of the paperaxis, and the printhead is again scanned back across the page, in theopposite direction. The printer may or may not jet ink during theretrace scan as determined by the print algorithm, which is designed tooptimize and trade off print quality and speed. The index amount may beless than the full Swath Height, allowing interlacing of the inkdeposited on the page to hide line feed errors and errors created bymissing or mis-directed nozzles, as determined by the print algorithm.The print rate is determined by the mechanics of scanning the Printhead,and is fixed by primarily the following factors:

-   -   A. The Swath Height;    -   B. The scanning rate (velocity that the printhead traverses the        page);    -   C. The number of interlaced scans necessary to achieve the        desired image quality;    -   D. The overtravel of the printhead—which is the width of the        active region of the print head or print head assembly; and    -   E. The acceleration and deceleration rate of the printhead.

Inkjet printer manufacturers have struggled to get the highest printrate (pages per minute), at the lowest manufacturing cost, while stillmaintaining the highest quality.

To date there have been two approaches to improving print speed:

-   -   A. Adjusting parameters of the factors associated with scanning        heads, described above; and    -   B. Using page width arrays.

But, there is difficulty in improving scanning heads because the keyparameters which affect speed are near their practical limits. Forexample:

-   -   A. Swath height: It is difficult to make swath height much        larger than 1 inch because the paper underneath the Nozzle Array        cannot be held flat enough in a region wider than that to avoid        ‘head crashes’, i.e., where the printhead, as it moves, hits the        paper. The inability to maintain a constant separation of the        paper from the printhead can be the result of imperfections in        the manufacturing tolerances of the printer, internal stresses        in the paper which tend to bow it, or “cockle” which is bending        due to swelling of the paper fibers as a result of absorption of        the water in the ink. Swath Height can be increased by using        either a printhead with a larger swath height (which are        expensive to produce) or two or more printheads offset in the        paper-axis direction to provide the effect of a single cartridge        with wider swath height. Both implementations are subject to        problems with flatness of the paper in the flat zone, among        other issues, and have not found much commercial use. Examples        of the staggered heads in the vertical (paper axis) direction is        found in U.S. Pat. No. 5,376,958 by Brent Richtsmeier and U.S.        Pat. No. 6,460,969 by Pinkernell for application to small format        printers.        -   FIG. 1 (prior art) shows a cut fed sheet 100, with a            printable region 102. The printhead ganged set 108 is shown            here without the carriage to be able to show the relative            position of the various components more clearly. Printhead            ganged set 108 as shown here includes printheads 106 K′,            106Y, 106K, 106C, and 106M. Each of the printheads 106            includes a nozzle array 104 that is shown as a rectangle            within the outline of each printhead (such a nozzle array            would only be visible from the bottom of the printhead            however it is shown here to illustrate the functioning of            the inkjet printer). As printhead ganged set 108 scans the            page it can be seen from FIG. 1 that printhead 106K′ is            positioned exactly one row higher than, and staggered to the            left of, printhead 106K with staggered black cartridges            106K′ and 106K effectively simulating a single long nozzle            array comprised of two nozzle arrays 104 as printhead ganged            set 108 scans across the page. This can be visualized by            mentally transposing printhead 106K to a position just below            printhead 106K′ where it could be seen that the bottom edge            of nozzle 104 of printhead 106K′ is substantially aligned            with the top edge of nozzle 104 of printhead 106K.        -   The technique of vertically staggered printheads has been            used in wide format printers such as the MacDermid            Displaymaker X-12™ printer at the cost of wider            paper-to-printhead separation, and hence lower print            quality.    -   B. Scanning rate: Scanning rate is determined by the formula:        scanning rate (ips)=firing frequency/dpi (where ips is inches        per second and dpi is dots per inch, the printer resolution).        Scanning rates above about 30 inches per second used for 600 dpi        printheads imply higher frequencies than can be supported by        printhead hydraulics (e.g., maximum ink flow rate) and power        limitations, especially since to increase quality, the desired        number of dots per inch have also been increasing. One attempt        to increase the scanning rate for black uses a second black        cartridge as described by Vilanova et. al in U.S. Pat. No.        6,471,332 B1, which is incorporated in its entirety by reference        here. Vilanova uses two adjacent black cartridges, firing        alternately, to achieve higher scanning rates without exceeding        the hydraulically limited frequency for the cartridge, and does        succeed in increasing the possible scanning velocity. However,        this scheme results in only minor speed improvements for office        printers because the acceleration and deceleration times        required to reverse direction and the increased overtravel from        the addition of the second black printhead almost completely        offset the increased scanning speed.    -   C. Also, higher scan rates require higher print head        accelerations which necessitate the inclusion of large expensive        motors to reverse the direction of the printhead at the ends of        the scan lines in a short amount of time, results in annoying        printer vibration.    -   D. Overtravel (i.e., the extra distance that printhead ganged        set 108 must travel for all of printheads 106 in the set to move        beyond the printable area of the page) cannot be reduced below 0        inches, and in any case, does not matter much once it gets        significantly below the width of the printable region 102.    -   E. Interlacing has been dictated by the inability to cover up        paper advance errors and missing or mis-directed nozzles. Little        progress has been made to date in completely eliminating missing        or mis-directed nozzles, although some technologies are better        than others.

On the other hand, page width arrays are expensive because they requirefull page widths of silicon heads for each of at least 4 colors (4×8.5inches=34 inches of silicon currently costing at least $20/inchmanufacturing cost), and the heads are sensitive to particle defectsbecause there is no way to interlace scans as is done with scanning headprinters. Thus to make an effective page width design requires redundantsets of nozzles, meaning, in reality, a viable page width array wouldprobably require at least 3 complete sets of 4-color page width nozzlearrays. This is a very expensive proposition, and not viable forprinters that are intended to sell for under a few hundred dollars.

Therefore, for black print speeds between about 18 pages per minute(achievable with 1 inch high printheads) and 100 pages per minute(achievable with page width arrays) for small format printers, therehas, to date, been no cost effective inkjet solution. For purposes ofthis discussion, all print speeds discussed are actual print speeds fora full page of text, which is typically much slower than “specificationspeeds” quoted-by-manufacturers on easy-to-print, sparsely covereddocuments. Although, today there are inexpensive printers that claim “20pages per minute”, this is for a sparsely printed page, typicallyprinted in a fast, and lower quality, draft mode).

Large format printers also have reached speed limitations. They havelarge flat zones which limit the resolution that can be accuratelyprinted. Further increases in print speed at high quality require anadvance in the state of the art.

To achieve high quality, there are printers that interlace their scansby factors of 3 or more to hide line feed errors, or missing nozzles.This means that they advance the paper only ⅓ of the swath height, orless per scan—resulting in 3 times or more the total page printing timethan would be the case if each print swath could be butted against thefollowing swath with little overlap.

The present invention optionally incorporates a scheme for buttingregions printed by different printheads without visible lines, gaps, orchanges in color. Past schemes include those in two patents: U.S. Pat.No. 6,357,847 assigned to Xerox which describes using a zigzag borderbetween the regions, and does not fully hide the borders; and U.S. Pat.No. 6,033,048 assigned to Hewlett Packard which describes ashingling/interlacing scheme for hiding line feed errors which slowsprinting, is inappropriate for butting of vertical regions, andincompletely hides variable overlaps.

Each of the shortcomings of the prior art noted above, as well asothers, are overcome by the present invention.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to significantly increase printspeed of inkjet printers, without significantly increasing their cost.Further, the present invention enables this improvement by usingexisting prior art components, whenever possible, and provides a savingof $100s of millions in manufacturing tooling costs. It is a furtherobject of the present invention to increase the quality of segmentedpagewidth array printers by eliminating print defects arising frommechanical tolerances at printed segment boundaries.

Further objects and advantages of the present invention will becomeapparent from a consideration of the drawings and the ensuingdescription.

The present invention is a method and an apparatus for increasing theprinting speed by subdividing the printed substrate into 2 or moredisjoint regions (except for, where appropriate, a small overlap region)which are printed with printheads or printhead arrays dedicated to eachregion, optionally using novel print algorithms that hide buttingerrors.

A further use of the novel print modes of the present invention is inthe hiding horizontal artifacts which result from errors such as sheetfeed errors. Use of this mode in the paper axis direction (the directionthe paper moves in a sheet fed, or continuous web printer) obviatespartial line feeds and shingling which would otherwise be needed to hidethose errors, resulting in another speedup factor.

The present invention further enables simultaneous high speed doublesided printing, greatly speeding up the printing of double sideddocuments, which would otherwise have to be produced by collecting thedocuments printed on one side, then flipping them over and running themthrough the printer a second time.

BRIEF DESCRIPTION OF THE DRAWINGS

NOTE: In several of the figures the printhead arrays are shown withoutthe mechanical details of the printhead transport mechanism to alloweasier visualization of the functioning of the prior art and the presentinvention.

FIG. 1 shows a simplified top view of a prior art scanning head of aninkjet printer aligned with a substrate (e.g., sheet of paper) to showits orientation therewith and the paper path, illustrating priorattempts to improve print rate;

FIGS. 2A and 2B show simplified top views of a prior art scanning headinkjet printer print mechanism, illustrating the initial and finalpositions of a printhead array;

FIGS. 3A and 3B show simplified top views of positions of the prior artprinthead array when the inkjet printer is only printing black text;

FIG. 4A shows a simplified top view of a scanning head of an ink jetprinter of the present invention incorporating a first embodiment of thepresent invention for positioning the scanning inkjet print mechanismrelative to a substrate (e.g., sheet of paper) in a scan starting orending position at the left side of the printed region of the substrate;

FIG. 4B is similar to and complements FIG. 4A also incorporating thefirst embodiment of the present invention, showing the scanning inkjetprint mechanism relative to a substrate in a scan starting or endingposition at the right side of the printed region of the substrate;

FIG. 5 shows a magnified view of the butting of a left and right raster(or columns, as shown in FIGS. 4A and 48), with a 50% dense halftonedimage, and a 2 pixel horizontal offset and 2 pixel vertical offsetbetween the left and right rasters. The image is 200 pixels wide by 80pixels high, and is at about a 20× magnification of a 600 dpi greyimage;

FIG. 6 shows the same conditions as in FIG. 5 with a 90% dense halftoneimage;

FIG. 7 shows the resultant image with the right and left rastersoverlapped in a region 40 dots wide using a partial embodiment of thepresent invention whereby there is a gradual replacement of dots by arandomly generated, linearly increasing mask function;

FIG. 8 shows the result when the same algorithm as used in FIG. 7 isapplied to a 90% dense halftone image as in FIG. 6;

FIGS. 9 A-D illustrate the use of a replacement algorithm of the presentinvention that uses two nonlinear functions, Alpha l (x, F), and Alpha r(x, F) for the right and left rasters of the original images as in FIGS.6 and 8. In FIG. 9A the horizontal offset is 0.4 dot increments; in FIG.9B the offset is 0.8 dot increments; in FIG. 9C the offset is 1.2 dotincrements, and in FIG. 9D the offset is 1.6 dot increments;

FIGS. 10 A-E illustrate the use of the same algorithm of the presentinvention applied to the same 90% dense halftone image as in FIGS. 6, 8,and 9. In FIG. 10A the horizontal offset is 0.0 dots. In FIGS. 10B-E thehorizontal offsets are increased by 0.4 dot increments from the previousone;

FIGS. 11A-E are plots in perspective that show iterations of thefeathering function Alpha l (x,F);

FIG. 12 is a perspective graph of Alpha l (x,F)+Alpha r (x,F) plottedagainst the scan axis position, x, in units of pixels and color densityvalue, F. The transition region is between pixel locations 60 to 140, 80pixels wide;

FIG. 13 is similar to FIG. 12 showing Alpha l (x,F)+Alpha r (x,F), whenthe pixel dots are not completely opaque. For this plot, the assumptionis that the incremental absorption of a second dot on top of an existingpixel dot would increase by 50%;

FIG. 14 is a typical curve of optical density versus dot density showingsaturation for black dots with a radius just sufficient to cover a pagecompletely at 100% dot density;

FIG. 15 is a magnified segment of test patches to be used to measureAlpha l, Alpha r pairs of a printer to be characterized;

FIG. 16 is a flow chart illustrating the method steps of the presentinvention for butting printed regions on the substrate without leavingvisible artifacts;

FIG. 17 shows a third preferred embodiment of the present invention in asimplified top view of a black only printer, with 3 black printheadsscanning 3 separate corresponding regions or columns of the substrate;

FIG. 18 shows a simplified perspective view of a first alternative ofthe third embodiment of the present invention, with the printheadsmounted rigidly together;

FIG. 19 shows a perspective view of the first alternative of the thirdpreferred embodiment, with the addition of a service station under thepaper path;

FIG. 20 shows a simplified perspective view of a second alternative ofthe third embodiment of the present invention with the printheadsmounted individually and connected to either the front or the back ofthe drive belt (shown here connected to the front of the belt);

FIG. 21 shows a simplified perspective view of a third alternative ofthe third embodiment of the present invention, with the printheadsmounted individually, and controlled individually by separate drivebelts, a novel paper drive mechanism;

FIG. 22 shows a simplified perspective view of a fifth preferredembodiment of the present invention to permit black printing incorresponding regions; on both sides of the paper;

FIG. 23 shows a simplified perspective view of the fifth preferredembodiment of the present invention to permit color printing on bothsides of the paper with double the black print speed over the firstpreferred embodiment, shown in FIGS. 4A and 4B;

FIG. 24A shows a simplified top view of a sixth preferred embodiment ofthe invention, with segmented pagewidth printheads;

FIG. 24B shows a perspective magnified view of the right end of thesegmented pagewidth printhead shown in FIG. 24A;

FIG. 25A shows the prior art interlacing technique to hide line feederrors;

FIG. 25B shows the sixth preferred embodiment of the invention where thefeathering method of the present invention is used to hide line feederrors;

FIG. 26 shows a simplified block diagram and drawing of a printerelectronics and paper path;

FIG. 27 shows a simplified top view of the printheads in a 4 colorprinter capable of printing 2 regions simultaneously;

FIG. 28A shows the function Alpha l in a region that includes atransition region;

FIG. 28B shows the function Alpha r in the same region as depicted byFIG. 28A;

FIG. 28C shows the sum of the Alpha l and Alpha r functions in the sameregion as depicted by FIGS. 28A and 28B;

FIG. 28 D shows the dots printed by a left printhead using the Alpha lfunction of FIG. 28A and in the same region depicted by FIG. 28A;

FIG. 28 E shows the dots printed by a right printhead using the Alpha rfunction of FIG. 28B and in the same region depicted in FIGS. 28A and28B; and

FIG. 28F shows the superimposed printing of the dots in FIGS. 28D and28E.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The embodiments described herein pertain to printing, including and notlimited to cut sheet, or web feed formats; small format home printers tohigher speed office printers; large format printers; fabric printers;and package markers, inkjet or laser; scanning head or fixed head.Printing also includes the application of material other than ink, forinstance biological markers, reagents, catalysts, etc.

Inkjet printing will be used as an example, although the presentinvention applies to other forms of “printing” as well.

The invention discussed below relates to primarily to scanning headinkjet printing, and also any other form of printing where adjacentprint regions may be formed to increase print speed, including printingwith Segmented Pagewidth Array heads, which are fabricated from segmentsbutted together to span the width of a page.

The embodiments of the present invention have been designed with thepurpose of increasing inkjet printing speeds over those of the prior artwithout sacrificing quality by incorporation of one or more of thefollowing:

-   -   A. subdividing the substrate into multiple regions each with its        own printhead or printhead arrays and optionally employing        appropriate novel print modes to hide raster spacing errors;    -   B. reducing the amount of interlacing through the use of the        appropriate novel print modes;    -   C. bidirectional printing;    -   D. printing on both sides of the document simultaneously.

A first preferred embodiment of the present invention increases the fullpage, high quality mode black text print rate of a printer over thespeed of prior art ink jet printers by about double without incurringmuch additional cost.

FIGS. 2A and 2B show conceptually a four color printer of the prior art,looking down on the 4 printheads (Traditionally Cyan, Magenta, Yellowand Black, CMYK as indicated on printheads 106Y, 106K, 106C and 106M) ina printhead array. When the printheads 106 Y, K, C, M are scannedlaterally to the right, ganged together from position 110L, they aresaid to scan over page (or substrate) 100 in a print swath. At the endof the scan on the right side of page 100, the printhead assembly passesbeyond the printable region 102 by the width of the head assembly(overtravel), plus an additional distance for the printhead todecelerate to position 110R as in FIG. 2B. Thus the complete_swath_timefor prior art printers to move from position 110L to position 110R isthe sum of four measurable transit times, namely:T _(swath)=printed width/velocityT _(overtravel)=2·overtravel distance/velocityT _(deceleration)=velocity/accelerationT _(acceleration)=velocity/acceleration  (Equations 1)with T_(swath) being the time to transit printable region 102;T_(overtravel) being the time to transit the overtravel distance with afactor of two being applied since there is an overtravel distance to beaccounted for on both the left and right sides of printable region 102;T_(deceleration) being the time to transit the distance beyond theovertravel distance to decelerate to position 110R in this example; andT_(acceleration) being the time to transit the distance from the pointof rest before the scan, point 110L in this example, to the beginning ofthe overtravel distance transited on the previous scan to decelerate toposition 110L.

The time to print an entire black only page is approximately the numberof swaths per page times the complete_swath_time, plus the paper advancetime (if it isn't advanced during the acceleration and/or decelerationtimes). When printing pages in color with prior art printers, often eachswath is repeated multiple times in an interlaced fashion to reduce thevisibility of paper advance errors or missing nozzle defects, and toprovide more subtlety in dither patterns. Thus for color pages theinterlace factor is typically 3 to 8 as opposed to 1 when printing blackonly, thus the print time for color printing is longer than for blacktext because the increased interlace factor is greater; becausebidirectional printing results in different shades depending on theprint direction (hence unidirectional printing is used, and the retracetime is wasted); and because of the overtravel associated with the 4colors of the printhead is much larger, and contributes still more tothe complete swath time.

Generally, in scanning head inkjet printers, increasing swath height andprinting velocity are the highest leverage variables to increasing printspeed, but they are also extremely expensive to change, since theyrequire very expensive tooling changes for the printer and theprinthead, or, in the case of swath height, expensive silicon areachanges to the printhead. Another design variable of importance is theovertravel, which must be at least the distance separating all theprintheads (i.e., distance from the nozzle array of the left mostprinthead to the nozzle array of the right most printhead). If the2·overtravel is comparable to the printed width, then about as much timewill be spent in non-printing motion as in printing motion. Therefore itis desirable to reduce 2·overtravel distance to much less than theprinted width.

Most of the embodiments below employ a feathering technique to make theboundary between regions printed with separate printheads, or at adifferent times, not visible to the eye. The feathering technique is asfollows:

-   -   A. Adjacent printable regions are slightly overlapped in a small        transition region.    -   B. In the transition region, the dots to be printed are a        combination of dots from the two printheads, one for each of the        adjacent print regions on the sheet. When there are only two        print regions on the sheet they are the left print region        printhead and the right print region printhead (top and bottom        printheads in the case of horizontal transition regions).    -   C. The fraction of dots in the transition region printed from        the left print region printhead are gradually decreased (as        viewed from left to right) according to a formula, analytically        or experimentally derived, as part of a method, as described        below in the second embodiment.    -   D. The fraction of dots in the transition region printed from        the right print region printhead are gradually increased (as        viewed from left to right) according to a similar formula,        analytically or experimentally derived, as part of a method, as        described below in the second embodiment.    -   E. Generally, in the transition region, the fraction of dots        printed by the left print region printhead plus the fraction of        dots printed by the right print region printhead exceed a total        of 1.0 (i.e., exceed the number of dots that would have been        printed had the region been one continuous region that was        printed by a single printhead all at once).    -   F. The dots printed in each of the left and right print region        are printed using any halftone mask with the mask being        different for each of the left and right print regions. The two        masks are designed to be uncorrelated—which has the practical        effect that changing misalignments between the left and right        print region printheads do not result in different densities to        the eye. The simplest way to generate uncorrelated halftone        masks is to base them on pseudo random numbers generated with        different seeds.

The feathering technique specified above is illustrated in summary inFIGS. 28 A-F. In these figures, a 200 pixel wide portion of a 4800 pixelwide page spanning a transition region with uniform density of 0.9 is tobe printed with the printable region of a sheet that is divided into twovirtual printable columns with a narrow transition between the twocolumns using a left printhead and a right printhead (see FIG. 4A), witheach of the left and right printheads each disposed to print in anoverlap region (i.e., the transition region) that is 80 pixels wide(from the 60 to 140 pixel position as depicted in FIGS. 28A-C). Apreviously determined feathering function Alpha l (illustrated in FIG.28A) for use with the left printhead determines what fraction of the 0.9density is to be printed with the left printhead as a function ofposition in the transition region. Similarly, a previously determinedfeathering function Alpha r (illustrated in FIG. 28B) for use with theright printhead determines what fraction of the 0.9 density is to beprinted with the right printhead as a function of position in thetransition region.

It can be seen from FIG. 28A that Alpha l (i.e., the fraction of dotsprinted by the left printhead) remains constant in the left columnprintable region where only the left printhead can print, whereas in thetransition region Alpha l decreases, eventually to zero, when thetransition region is viewed from left to right. Similarly, it can beseen from FIG. 28B that Alpha r (i.e., fraction of dots printed by theright printhead) increases from zero in the transition region whenviewed from left to right to a constant level in the right columnprintable region where only the right printhead can print, substantiallythe inverse as for the left printhead. (It needs to be kept in mind thatthe functions Alpha l and Alpha r are not necessarily the exact inverseof each other since they will not be the same as stated in F. of theabove discussion of the feathering technique.)

In FIG. 28C the sum of the Alpha l and Alpha r functions is plotted,where it can be seen that in the transition region the sum of the twofunctions is greater than either of the Alpha functions individually inthe regions where only the one printhead can print as stated in E. ofthe above discussion of the feathering technique.

FIG. 28D illustrates the resultant dot pattern printed with the leftprinthead in the pixel region depicted in FIG. 28A (that includes thetransition region), using a halftoning algorithm based on a pseudorandom mask that is the result of halftoning 0.9·Alpha l.

Similarly, FIG. 28E illustrates the resultant dot pattern printed withthe right printhead in the same pixel region as also depicted in FIG.28B (that includes the transition region), using a halftoning algorithmbased on a different pseudo random mask that is the result of halftoning0.9·Alpha r.

When the resultant dot patterns from the left and right printheads asshown in FIGS. 28D and 28E are superimposed on the page, the resultantpattern is that in FIG. 28F. This is a somewhat surprising result sincethe fraction of dots in the center of the transition region, as shown inFIG. 28C, is 1.5 times the number of dots that would have been printedto obtain a similar density in a region that is not in the transitionregion.

In a normal image, the to-be-printed image is non-uniform. Nevertheless,the procedure is the same. At every position, multiply the target imagedensity by the appropriate Alpha l (for the left printhead) and Alpha r(for the right printhead); and halftone the result with uncorrelatedmasks for the left and right regions. Alpha l and Alpha r are functionsof both the local density and the horizontal position in the transitionregion as will be seen in the following discussions.

First Embodiment Black Single Sided Printing with Two Print Columns

The first preferred embodiment of the present invention to be describedbelow enables the use of existing technology printheads, while at thesame time increasing the black print speed by a factor of 2 over that ofthe prior art configuration shown in FIGS. 2A and 2B; and FIGS. 3A and3B in a simple, and novel way. Variations and combinations of thepresent invention described below can increase print speed by a factorof 3 to 96 times over prior art implementations, at little cost increaseover that of the prior art.

The various additional preferred embodiments use additional printheads,or additional printhead arrays, to print on the page, dedicating variousprintheads or printhead arrays to corresponding parts of the page. Thetechnique of the additional embodiments is most useful when theovertravel time (T_(overtravel)) plus the acceleration and decelerationtimes (T_(acceleration)+T_(deceleration)) of the printhead, or printheadarray, is much less than the printed swath time (T_(swath)) for eachprinthead assembly.

The first preferred embodiment of the present invention is exemplifiedby the following implementation which is applicable to printers thatprint on typical office paper (e.g., 8½″×11″ paper), and is easilygeneralized to any scanning head inkjet printing system, whether it jetsfluid on paper, fabric, biological materials or other substrates.

If the usual prior art print head array is built with 4 printheads on0.75 inch centers in a YKCM order, the first embodiment of the presentinvention is equipped with 5 of the same type printheads on 0.75 inchcenters, with a printhead order of KCYMK (i.e., with the blackprintheads on the ends of the printhead array). This would have aspacing between the black printheads of 4×0.75 inch=3 inches, which isabout half the normal printed width on business documents in printableregion 102.

For example, FIG. 2A (prior art) shows the printable region 102 of a 4color inkjet printer schematically, with the 4 color heads 106Y, K, C, Mfully off the left edge of the paper 100. The width that the printheadarray in position 110L extends past the edge of the paper width, theovertravel distance, is equal to the width of the printhead array nozzleregion.

FIG. 2B (prior art) shows that same print head array at position 110Rafter having printed a swath, and momentarily at the right end of itsscan. In color printing, the complete printhead array must extend beyondthe printable area 102.

However, if this same printhead array is printing only black text, thenthe printhead array does not have to scan as far as shown in FIGS. 2Aand 2B, rather the left and right positions on black only printing inthe prior art would be as shown in FIGS. 3A and 3B (prior art). In theblack only printing case, the printhead array at position 110LK in FIG.3A need only travel between the margins of page 100 to position 110RK,and, more importantly, there is no additional overtravel required asthere is with respect to color printing. The scanned printhead arraycarriage travel distance equals the width of printable region 102 ofpage 100, plus a small amount required to accelerate the printhead arrayto its printing speed, which is dot frequency/dpi (i.e., dots per inch),which is typically 18,000 dots per second/600 dpi, or 30 inches persecond (ips).

FIGS. 4A and 4B illustrate the first preferred embodiment of the presentinvention. In FIG. 4A the printhead array includes printheads 106K′,106C, 106Y, 106M and 106K, in that order from left to right, where 106Kand 106K′ are equivalent black printheads, and that array is located atposition 112L with black printheads 106 K′ and 106K on each end,approximately 3 inches (center-to-center) apart from each other. Whenprinting black text in a region that is about 6 inches wide, as would betypical on an 8.5 inch wide piece of office paper 100 with about 1 inchmargins, it is necessary for the printhead array of the first embodimentof the present invention to move only 3 inches (plus a small additionaldistance to accelerate the printhead array) to print a full swath acrossprintable region 102. FIG. 4A shows printhead 106K in the left marginjust outside printable region 102 with the printhead array at position112L at its left extreme position compared to the printable region 102;while FIG. 4B shows printhead 106K′ in the right margin just outsideprintable region 102 with the printhead array at position 112R at itsright extreme position compared to the printable region 102. In viewingFIGS. 4A and 4B it can be seen that the distance the printhead array hastraveled, with black printhead 106K′ having printed region 114L ofprintable region 102 and black printhead 106K having printed region 114Rof printable region 102, and both printheads 106K and 106K′ havingprinted in overlap region 116 in the center of printable region 102.Since, in this example, overtravel time is approximately 0, andacceleration time is small compared to swath time, reducing theComplete_swath_time by almost a factor of 2 from that of the prior artreduces the overall printing time by a similar factor.

Thus the addition of a single black printhead by the first embodiment ofthe present invention to a 4 printhead array of the prior art, andchanging the printing algorithm so that each of black heads 106K and106K′ print on their respective sides of the paper rather than using asingle black printhead to print totally across the page can double theprint rate over that of the prior art, with little additional cost, andlittle printer, and no printhead redesign.

Alternatively, if the printer is only to be used for black printing (noother colors) printheads 106C, 106Y and 106M could be omitted whilekeeping black printheads 106K′ and 106K mounted on 3 inch centers asdescribed above which will present the same time savings as discussedabove with the 5 printhead configuration shown in FIGS. 4A and 4B.

In printing black text, the printer mechanism of the present inventionmay be built precisely enough that no printing gaps will be visible inthe overlap region 116 between left and right print regions 114L and114R. However, when printing a grey tone, overlaps or gaps may bevisible which can be overcome with the use of a novel print algorithmfor the printheads, described below for color printing, to hide anyerrors in printing in the overlap and transition region 116.

Second Preferred Embodiment Color Printing

The concept of the present invention can be generalized to colorprinting as well. To print color, one replicates the printhead arrays,so there is one array for each region of the paper. To be effective inimproving speed, each printhead array must be less than about half ofthe width of the subsection of the paper that it is designated to print,and printer acceleration should be fast enough so that printheadturnarounds (i.e., direction reversals) do not dominate print time.Thus, a page could be divided into 2 or more columns, each of which tobe printed by a corresponding print head array.

FIG. 27 shows a simplified top view of an inkjet printer for 4-colorprinting only on sheet 100 with margins 101 and printable region 102.Printable region 102 is shown schematically with two printable columns(114L and 114R) and a transition region 116 between the two columns.Additionally, there are two scanning printheads (115L and 115R) eachincluding four ink tanks and corresponding individual print nozzleplates 117 at the right most scan printing starting position. In each ofprintheads 115L and 115R the four print nozzle plates 117 are depictedas parallel vertical straight line segments. The left most print nozzleplate 117 of printhead 115L is substantially aligned with the right edgeof transition region 116 while the left most print nozzle plate 117 ofprinthead 115R is substantially aligned with the right edge of printableregion 102 (column 114R). Printheads 115L and 115R when in their leftmost printing starting position will have the right most print nozzleplate 117 of printhead 115L substantially aligned with the left edge ofprintable region 102 (column 114L) while the right most print nozzleplate 117 of printhead 115R is substantially aligned with the left edgeof transition region 116. In each scan all four print nozzle plates 117of each of printheads 115L and 115R pass over, printing when excited, intheir corresponding column 114L or 114R (i.e., left to left and right toright), plus all four print nozzle plates 117 of each printhead 115L and115R pass over transition region 116, printing when excited. An examplealgorithm the second embodiment of the present invention is described asfollows.

Printhead arrays exist today that use a variety of configurationsincluding:

-   -   A. Separate cartridges with their own heads for each of C, Y, M,        K (and possibly C, M′, G, O, K′ and K″ (here O refers to orange,        and G refers to green);    -   B. Separate K cartridge with integral printhead, and one or two        multi-color cartridges, one configured as C, Y, M and optionally        a second configured as C′, Y′, K′;    -   C. Single multi-color cartridge with C, Y, M, K and a single        printhead.

These prior art print head configurations may be either on-axis (meaningcontaining their own integral ink supply) or off-axis (meaning ink issupplied to the print head via some mechanism, typically a hose, from astationary supply).

Ideally, the cartridge and printhead array configuration would be chosenso the distance from the furthest apart nozzles in the printhead arrayis as small as possible, allowing the profitable use of more printheadarrays and corresponding paper regions, still meeting the constraintthat the printhead array overtravel is much less than the correspondingprint region width.

For the first and second embodiments discussed above, as well as otherembodiments discussed below, if the printer is only to print a fixedwidth printable region with a fixed number of printable columns, theprinthead arrays (one for each printable column) can be mounted a fixeddistance apart from each other on the same drive mechanism (e.g., adrive belt). To give the printer end user more flexibility and allow theuser to define different width printable regions, as well as differentwidth printable columns, each of the printhead arrays with which theprinter is equipped could be fixed to separate drive mechanisms (e.g.,multiple belts as shown in FIG. 21 which is discussed below). In eithercase, the operation will be the same as described.

In the second embodiment of the present invention, for 2 or morecolumns, or more generally, print regions, with corresponding black andcolor arrays assigned to printing those columns, or, more generally,regions. Print regions typically will be rectangular, or alternatelycould be chosen to have jagged edges to provide a butting region thatavoids difficult in hiding features such as high spatial resolutiondetail.

Theory of Operation for Color Printing

The key to the successful color implementation with the use of separateprintheads or printhead head assemblies for scanning correspondingregions is accurate color matching of the printhead assemblies togetherwith no apparent gaps, lines, or changes in color at region boundaries.The following describes design directions and calculation principles,and methods of the present invention which allow seamless butting of theprint regions. Though this butting algorithm is first discussed in thecontext of scanning head inkjet printers which are dividing up the pagein the scan axis direction, the same algorithm is also useful for:

-   -   A. minimizing line feed print artifacts in scanning head        printers—and therefore enabling less interlacing for a given        print quality, and therefore further increasing speed;    -   B. minimizing the butting errors in segmented head page width        arrays; and    -   C. minimizing moire effects in any print process were two layers        are successively printed on the same region. This could occur,        for example, in printing shingled color patterns bidirectionally        because the offsets of one set of color dots relative to another        from scanning in one direction would be different than the        offset between different color dots in the other direction.

Printheads and printhead arrays may be manually or automatically alignedso their corresponding adjoining regions appear nearly seamless, andprint algorithms adjusted to hide residual butting errors. The overallprocess for achieving highest quality results includes:

-   -   A. Subject to cost constraints, improve the mechanical accuracy        of the carriage scan and paper advance mechanisms, and improve        the color accuracy and stability of the individual printheads;    -   B. Correct as much residual positional or drop volume inaccuracy        as is economically reasonable with a feedback mechanism (manual,        or automatic and built in), subject to cost constraints; and    -   C. Use printheads and print algorithms that, for color, use 2 or        more of the same color dots on a paper pixel to obtain full        saturation of that color (this is called multilevel printing).        Alternatively stated, for each pixel and each color, have an        inherent density of at least 3 levels (0, 1, or 2 drops on the        pixel). The more levels used, the less image noise and butting        artifacts achievable.

Because the mechanical tolerances of the printers, even with feedback,cannot economically be made so that the raster printed by the leftprinthead array is perfectly aligned with the raster of the rightprinthead array to less than, say 1/10 dot diameter in both the scan andprint axis, a transition region is needed to feather from printing theleft raster to printing the right raster. By experimentation it has beendetermined that the transition should be 10 to 40 times the width of themaximum raster error anticipated. For example, for 600 dpi resolution,with 1 dot raster residual placement error, the transition region wouldbe 40 dots at 600 dpi, or about 40/600 or about 0.067 inches wide. Inthat transition region, a pixel or dot that would have been printed inthat region could be printed by either the left printhead array, or theright printhead, or both, as described below.

To minimize the effects of any remaining inaccuracy,

-   -   A. optionally, if possible, define the boundary between the left        and right printed regions to areas that are insensitive to        placement errors of the right and left (for example, all white        areas), while still keeping the left and right print regions        approximately the same width. For example, black text on a white        background would be printed by shaping the print zones boundary        to go between the letters;    -   B. hide print artifacts through the use of algorithms that        spread the error through the transition region in a way to which        the human eye is not sensitive.

Therefore ways to improve color uniformity through the transition regioninclude various combinations of the following:

-   -   A. Use of very accurate and stable lead screw mechanisms and or        carriage position feedback mechanisms such as optical position        encoders, providing accurate head positioning on the order of a        few dots;    -   B. Use of manual or automatic sensing of printing errors on test        substrates, and feeding back color corrections to the print        mechanism tables and formatters;    -   C. Use of interlacing patterns, and overlapping scans;    -   D. Use of dither patterns or other color mask algorithms with        little short range order;    -   E. Adjusting the print overlap region boundary so print overlap        regions do not include hard to hide overlaps. In general, this        would mean performing the overlap if possible in regions of less        than 70% color saturation if possible, and in areas of        relatively low spatial frequency content;    -   F. Use vertical shifting of the patterns to compensate for        printhead vertical alignment:        -   a. In the case of partial row offsets (vertical offsets) use            a combination of:            -   i. Dot size modulation (when available);            -   ii. Number and placement of dots (dither and error                diffusion patterns);            -   iii. Use Additional colors (C′ and M′) to fill in;    -   G. Use “showerhead” nozzles with built in offset tolerant        patterns, such as discussed in U.S. Pat. No. 6,354,694;    -   H. Use of process grey instead of grey comprised of black dots        on a white background;    -   I. Use multiple dots on a single resolution element to make a        single saturated color, thereby allowing lower image noise, and        providing more color values, and hence more flexibility in        matching overlapping region colors to adjacent regions.        Print Algorithms

Generally the details of the method used depends on:

-   -   A. The resolution and positional accuracy of the carriage        mechanism, and the, color accuracy and stability of the        printheads used initially;    -   B. The ability to correct for positional accuracy errors either        manually or automatically;    -   C. The pattern to be printed;    -   D. The number of colors available in the printhead assembly.

After having chosen a method for minimizing the offset of the rightraster from the left raster, and matching the color tables of the rightand left printheads so that they each print the same colors throughoutthe color range, the final step is the feathering process (a method ofthe present invention) which hides any residual raster offsets,regardless of what causes those offsets.

In summary, the feathering algorithm:

-   -   A. Defines a small transition region between the right and left        rasters where the two rasters overlap each other;    -   B. Calculates two feathering functions of raster position,        called Alpha l (Alpha for the left raster) and Alpha r (Alpha        for the right raster) which determine the value to be printed by        the corresponding printhead or printhead array at each point.        Those values are F(x,y)·Alpha l(x,F) [where F is the desired        image density] and F(x,y)·Alpha r(x, F). In this and the        discussions that follow, x is the scan axis direction and y is        the paper advance axis direction;    -   C. Use two uncorrelated stochastic dither patterns (one for the        left raster, and one for the right raster) to half-tone the left        and right raster pixel patterns with the desired values.

To illustrate the feathering algorithm for a light grey background colorthe fraction Alpha l of dots printed by the left printhead is graduallydecreased from 1 to 0, while the fraction Alpha r of dots printed by theright printhead is increased from 0 to 1, symmetrically progressing fromleft to right across the transition region. The fraction of dots chosenfrom each raster changes gradually with the corresponding Alpha, witheach individual dot printed or not printed according to a randomhalftoning mask variable chosen to have the average density at thatpoint of the target fraction F·Alpha l or F·Alpha r. An unexpectedresult is that the sum of Alpha l and Alpha r generally add up to morethan 1. For black densities over about 0.80, the fraction of black dotsprinted by the left printhead, and the fraction printed by the rightprinthead, at a given spot, could add up to almost 2 with some spotshaving 2 dots printed, based on a table of Alpha l (x, F) and Alpha r(x, F) generated either experimentally or analytically for each printerfamily.

It is important that the random variable chosen as a mask variable fordeciding when to print dots from the left array be different, anduncorrelated to, the random mask variable chosen as a mask variable fordeciding when to print dots from the right array as was discoveredexperimentally and verified by mathematical analysis. Practically, thisis most easily done by using different seeds for the pseudo randomnumber generators that generate the random mask variables for the leftand right rasters. Using uncorrelated masks makes any density or colorshift that might occur in the transition region completely independentof any misalignment of the raster scans in the two regions. Therefore,the color correction process in the transition region does not have torely on knowing the offset of the rasters, or that the offset be stablewith time.

FIG. 5 shows the enlarged image of an 80 pixel high by 200 pixel wideprinted area region of a 50% dense halftone image when two adjacentrasters 122L and 122R and are butted with a 2 pixel offset in the xdirection and a 2 pixel offset in the y direction, without using thenovel feathering scheme of the present invention. An unwanted white linecalled a band is visible from gap 124.

FIG. 6 shows the enlarged image of a 80 pixel high by 200 pixel wideprinted area of a 90% dense halftone image when two adjacent rasters122L and 122R are butted with a 2 pixel offset in the x direction and a2 pixel offset in the y direction, without using the novel featheringscheme of the present invention. Again, a considerable band is visiblefrom the visual perception of the gap 124.

FIG. 7 shows an enlarged image of a 80 pixel high by 200 pixel wideprinted area with a 40 pixel wide transition region 124 of a 50% densehalftone area with a 2 pixel offset in the x direction and a 2 pixeloffset in the y direction feathered using a linearly decreasing fractionof pixels from the left raster, and the same linearly increasingfraction of pixel in the right raster. In this image, the randomvariable used to implement the mask in the transition region 124 is thesame for the left and the right raster. This image is much improved fromthe corresponding image in FIG. 5, however, the solution employed heredoes not work well for all raster offsets and for all image densitieswhich will be seen in the discussion of FIG. 8.

FIG. 8 shows the enlarged image of a 80 pixel high by 200 pixel wideprinted area with a 40 pixel wide transition region bounded on the leftby 125L and the right by 125R, and encompassing (former gap) 124 of a90% dense halftone area with a 2 pixel offset in the x direction and a 2pixel offset in the y direction feathered using a linearly decreasingfraction of pixels from the left raster, and the same linearlyincreasing fraction in the right raster. In this image, the randomvariable used to implement the mask is the same for the left and theright raster with the image clearly showing unacceptably large amountsof “white” in the transition region 124.

FIG. 9A shows the enlarged image of an 80 pixel high by 200 pixel wideprinted area with a 40 pixel wide transition region of a 90% densehalftone area with a 2 pixel offset in the x direction and a 2 pixeloffset in the y direction feathered using a non-linearly decreasingfraction of pixels from the left raster, Alpha l, and a mirror imagenon-linearly increasing fraction in the right raster, Alpha r. The sumof these functions (Alpha l+Alpha r) add to a value that is greaterthan 1. In this image, the random variable used to implement the mask isthe same for the left and the right rasters. While this showsimprovement over the result illustrated in FIG. 8, the result is notconsistent with varying offsets used to obtain the results as shown inFIGS. 9 A-D. In FIG. 9A the offset is 0.4 dot increments; in FIG. 9B theoffset is 0.8 dot rows; in FIG. 9C the offset is 1.2 dot rows, and inFIG. 9D the offset is 1.6 dot rows.

FIGS. 10 A-E show the enlarged image of an 80 pixel high by 200 pixelwide printed area with a 40 pixel wide transition region of a 90% densehalftone image with a 2 pixel offset in the x direction and a 2 pixeloffset in the y direction (the toughest case) each processed with thepreferred algorithm of:

A. Two different nonlinear calculated functions (Alpha l and Alpha r)that add to more than 1; and

B. Uncorrelated random masks for each of the right and left rasters.

In FIGS. 10A-E, the offsets are 0.0 dots in FIG. 10A with the offsetincreasing by 0.4 increments and in each of FIGS. 10B-E. FIGS. 10A-Eshow almost indiscernible butting errors, over a full range ofmisalignment.

Determining Alpha l and Alpha r: Analytical Calculations for Alpha

Alpha l must transition from 1 to 0 as the left raster is feathered fromleft to right across the transition region, and similarly, Alpha r musttransition from 0 to 1 across the transition region. Initially, onemight assume that these could be linear transitions—howeverexperimentation and subsequent mathematical analysis has shown that thatis only the case when there is very low color saturation.

What in fact must be done is to gradually replace dots from the leftraster with dots from the right raster in the transition region. If therasters are aligned, it does not make a difference from which raster thedots came. However, if the rasters are misaligned by, for instance, 2pixels, when one dot from the left raster is not used, the dot that isintended to replace it from the right raster will land 2 pixels away,possibly on top of another dot from the left raster. Thus, the dot fromthe right raster will not contribute as much to the darkness of theregion as expected. To compensate for this effect, in high density(dark) regions, the left and the right rasters, together, must have morethan 100% of the dots in the transition region. By the nature of theoverlap of the rasters, the two rasters, in the transition region, cantogether provide as much as 200% dots if necessary.

The relationship between Alpha l and Alpha r at any scan axis position xmust be as follows:F=F·Alpha l+F·Alpha r·(1−F·Alpha l)  Equation 2AOr, alternatively,F=F·Alpha l+F·Alpha r−F ²·Alpha L·Alpha r  Equation 2B

Where:

-   -   F=the density desired in the region (pixels dark);    -   Alpha l=the fraction of pixels used from the left raster; and    -   Alpha r=the fraction of pixels used from the right raster.

While this gives a relation between Alpha l and Alpha r at each point inthe transition region, it does not yield what either is as a function ofposition. However, it is known that the functions are mirror images ofeach other, and Alpha 1 equals 1 at the left side of the transitionregion, and 0 at the right side of the transition region, while Alpha requals 0 at the left side of the transition region, and 1 at the rightside of the transition region. One way to determine the two functions isiteratively.

Initially it is assumed that Alpha l is a cubic function that has 0slope at the right and left edges of the transition region, and hasvalue of 1 at the left edge, and 0 at the right edge of the transitionregion. (Any monotonically, gradually decreasing function is areasonable starting point, however functions that start featheringgradually will not show a line at the transition region edge). Next,compute the corresponding Alpha r function. Although the initialassumption for Alpha l and the computed Alpha r satisfy therelationship, they would be asymmetric solutions. To obtain a symmetricsolution, flip Alpha r (computed above) around the transition regionmidpoint, and then, average the flipped function with the previousassumption for Alpha l. Call the average a new Alpha l. Iterate theprocess until the functions do not change. The function Alpha l is thedesired function, and its mirror image is Alpha r.

Iterations of the calculation for Alpha l (x, F) are plotted in FIGS.11A-E.

It has been experimentally found that the method discussed above doesnot always converge. Another analytical method B for determining Alpha l(x, F) and Alpha r (x, F) is as follows:

-   -   A. Obtain Alpha l (F) as a function of Alpha r (F) either by        equation (2A discussed above, or equation 3A to be discussed        below as appropriate), or measuring color test samples as        described later herein;    -   B. Fit a quadratic curve (which will be the left half of the        function Alpha I) to the following parameters:        -   a. The curve Alpha l (left edge, F) goes through x=left edge            of the transition and has value 1 and slope 0, so Alpha l            (left edge, F)=1; Alpha l′ (left edge, F)=0;        -   b. The curve goes through x=center of transition region so            Alpha l (center, F)=Alpha r (center, F);    -   C. This fitted curve is the left half of Alpha l, and is        uniquely specified by the 3 conditions above;    -   D. Compute the left half of Alpha r (x, F) from either equation        2 or 3, or measured data giving Alpha l (F) in terms of Alpha r        (F); and    -   E. Compute the remainder of Alpha l and Alpha r by recognizing        that the right half of Alpha l is the mirror image of the left        half of Alpha r, and the right half of Alpha r is the mirror        image of the left half of Alpha l.

One will recognize that this approach is one of many to generate a setof curves Alpha l (x, F) and Alpha r (x, F), and that other conditionson the initial selection of the left half of Alpha l may be appropriate.For example, instead of fitting a quadratic curve, one could choose acubic curve with the additional constraint that the second derivative ofAlpha l at the left edge of the transition region should be 0. Or, itcould be insisted that there be no discontinuities of slope of theAlphas at x=center. Generally, however, the choice of the quadraticcurve leads to imperceptible transitions. Finally, both assumptionscould be made, and a quadratic curve used to approximate the left halfof Alpha l.

FIG. 12 is a perspective graph of Alpha l (x,F)+Alpha r (x,F) plottedagainst the scan axis position (x) in units of pixels and color densityvalue (F) and the transition region is between pixel locations 60 to140, i.e. 80 pixels wide. The sum of Alpha l and Alpha r (Alpha l+Alphar) as a function of position and color density F is shown in FIG. 12,showing that the total ink used is more that 1 dot per resolutionelement, and higher amounts of ink are used at higher color densities(saturations) (larger F).

A modification of this procedure is required if the ink used is notopaque. In that case, if a second drop from the right raster lands ontop of a previous drop from the left raster, there will be anincremental contribution, Q, to color, and the equation relating Alpha lto Alpha r above is modified to:F=F·Alpha l+F·Alpha r·(1−F·Alpha l)+F·Alpha r·Alpha l·F·Q  Equation 3Awhich simplifies toAlpha r=(1−Alpha l)/((Alpha l+Alpha r·(1−Alpha l·(1−Q)))  Equation 3Bwhere the added term reflects the additional contribution of the F·Alphar pixels added from the right raster that land on pixels on the left(F·Alpha l) and contribute an additional darkening Q.

Equation 3A is useful for computing the Alpha functions when color inksare used, which are not completely opaque. FIG. 13 shows Alpha l andAlpha l+Alpha r with Q=0.5. Notice that, in the case of a larger Q, themaximum sum of Alpha l and Alpha r is 1.15, i.e., there is only a 15%duplication of dots.

It should be noted that some of the analyses above have been simplifiedto make them easier to understand, or to represent mathematically.Nevertheless, the conclusions are generally applicable.

For example, in equations 2A through 3B, that relate the desired colordensity F to Alpha l and Alpha r, a simplifying assumption was made thatthe density of the image was F, which was assumed to be linearlyproportional to the number of pixels per unit area. In fact, density isgenerally a saturating function of F, as shown in Figure 15.

Thus a more accurate equation than equation 3A would be:D(F)=D(Alpha l·F)+{1−D(Alpha l·F)}·(D(Alpha r·F)  Equation 4where D(F) represents “the density corresponding the dot density”, whichis not generally not represented only by F·Alpha.

From Equation 4, it is possible to solve numerically for a relationbetween Alpha r and Alpha l and F. Using that relationship, as doneabove, Alpha (x, F) can be derived.

Alternatively, and probably more directly and therefore more accurately,the relationships between Alpha l (F) and Alpha r (F) can be measureddirectly as described below. This avoids many approximations in therelations between dot density and image density.

The above method determining feathering functions, and usinguncorrelated masks are suitable for binary printing (where there areeither 1 or 0 drops of ink on a pixel). For black text, this is commonlydone today; however for color and frequently grey images, more than onedrop is frequently used on a pixel to gain more saturated colors, andless noise.

In most cases prior art printers interlace and “shingle” color patterns,especially in higher quality print modes. For example, to print a highdensity cyan color, a printhead would fire, for example, every thirdnozzle along the nozzle array at a time, throughout the horizontal scan,then index the paper by ⅓^(rd) swath height and scan a second time, andthen index and scan a third time (interlacing). At the end of thisprocess, one third of a full swath height would have been completelycolored in, and areas above that colored swath would have been partiallycolored in. Interlacing refers to the ⅓^(rd) swath advance; shinglingrefers to staggering which ⅓^(rd) of the dots in the rows are fired onthe printhead depending on the position of the printhead in the scanaxis direction. This prior art methodology tends to obscure line feederrors, and reduce the visibility of missing or misdirected nozzles. Inaddition, prior art printers often use multiple levels of color on asingle pixel (optionally putting more than one dot of a given color on apixel to reach a saturated color). This gives less apparent noise inmid-tones, and further reduces the visibility of missing or misdirectednozzles. Use of additional inks with smaller dye loads gives theopportunity for less visible noise.

In general, it is known that throughput can be increased if lessshingling and interlacing are done. For color images where the printheadresolution is greater than the eye's ability to detect the resolution,it is known in the prior art that by taking advantage of multiple dropson a resolution element to provide smoother looking color regions, andinkjet printers are trending to “multidrop” and “multiple dye loadedink” solutions today. These multiple levels, in the present invention,no matter how implemented, provide the opportunity for better colormatching using the algorithms discussed above as part of this invention.

One additional way known in the prior art to reduce color noise, whichprovides a multi drop effect, but with less granularity than simply highresolution, or lightly dye loaded multi drop—which may put too muchwater on the page is described in U.S. Pat. No. 6,354,694.

Use of all the above techniques known in the prior art are compatiblewith the present invention, and tend to further increase the efficacy ofthe algorithms of the present invention and reduce the visibility of thetransition region which the present invention minimizes substantially.

Feathering with Multilevel Color

When printing, for example, 3-levels (0, 1, or 2 droplets or dots on apixel), the prior art includes various options for establishing greyscale, typically called “halftoning”.

In the binary case, typically in the prior art a pseudorandom decimalnumber between 0 and 1 (usually a computed, or possibly a precomputed‘mask’ of random numbers corresponding to each position in the potentialpixel array) is added to the desired density F (also a number between 0and 1), and takes the integer part of the sum. If the result is 1, a dotis printed; otherwise, the dot is not printed. There are many variantsof halftoning known in the prior art using different methods ofgenerating the masks, such as those described in U.S. Pat. No. 6,543,871assigned to EFI, U.S. Pat. No. 5,726,772 assigned to RCT and U.S. Pat.No. 6,057,933 assigned to Hewlett Packard. These halftoning algorithmsgive a random binary output whose mean value is the desired density, andwhose variance (noise energy) isσ² =F·(a−F)·a  Equation 4where F is the desired grey value, and “a” is the level separation (1 inthe case of binary printing). Thus the maximum σ² occurs in themid-tones and is 0.5·0.5·1=0.25, for binary printing.

In the prior art for the case of 3-level printing a grey level can begenerated several ways, with different noise characteristics.

-   -   1. Given an F, one can choose 0, 1, or 2 droplets on a pixel. If        the target F is 0.6, one should select either 1 or 2 pixels at        random, however with the probability such that it is 4 times        more likely to pick 1 dot than 2 dots, giving an average value        of 0.6.    -   2. Alternatively, one can divide the target value (0.6) by 2 to        get a value for each of 2 binary planes for each of the rasters.        The planes would use the binary algorithm described above, each        using independent random variables to perform the dither. The        printer would print each of the 2 planes. The result would have        an average value of 0.6, however a larger σ.

Algorithm 1 as stated above has a maximum σ² (noise energy) occurring atdensities of ¼^(th) and ¾ths, of 1/32; Algorithm 2 as stated above has amaximum σ² (noise energy) occurring at densities of 0.5 of ⅛. Algorithm1 is generally preferred since it yields the lowest maximum noise. Inaddition, in the prior art, one can correct superpixel regions of pixelsthat have an average value different from the target value, and addcorrections to some of the pixels by error diffusion to achieve lowernoise for the superpixel areas. This would be the process if each colorplane is considered separately. It is also possible to generate ditheredcolor planes that are coordinated to reduce color noise. See for exampleU.S. Pat. No. 6,057,933, which is incorporated by reference.

It will be of interest later in the discussion of the present inventionto consider a halftoning algorithm that has the same noise contentindependent of the level. One such algorithm in the present inventionis:

-   -   A. Chose the three nearest levels, A, B, C (not the nearest two        levels, as is done above in the prior art) closest to a value V,        the value to be halftoned;    -   B. Select one of the three levels in A using a random mask        variable, subject to the constraints that:        -   a. 0≦P_(A), P_(B), P_(C)≧1;        -   b. The sum of the probabilities of A, B, or C occurring is            1; i.e., (P_(A)+P_(B)+P_(C)=1);        -   c. The expected value of the resultant is the desired level;            (A·P_(A)+B·P_(B)+C·P_(C)=V);        -   d. The noise power is set to the maximum noise that would            have occurred with the prior art halftoning            (P_(A)·(A−V)²+P_(B)·(B−V)²+P_(C)·(C−V)²=a³/4)    -    where P_(B) is, for example, the probability of selecting level        A; and “a” is the level separation.

Whatever algorithm in the prior art would have been used to generate thecolor planes in the case of a single region and printhead assembly canbe adapted to multiple heads/head assemblies and regions as follows.

The preferred algorithm for multilevel printing is similar to that ofbinary printing.

-   -   A. For each pixel in each color plane that would have been used        in the single region printing process, determine the        corresponding Alpha l(x,F) and Alpha r(x,F) values, in the        transition region and    -   B. multiply Alpha l(x,F) times the corresponding color value of        F (x) of the single region color plane, resulting in a target        local density for the left raster D I(x,F);    -   C. multiply Alpha r(x,F) times the corresponding color value of        F(x) of the single color pane, resulting in a target local        density for the right raster D r(x,F);    -   D. Using separate (uncorrelated) bilevel pseudorandom masks for        the right and left rasters, select among the two closest values        for each pixel corresponding to D.

In the case of more than 3 levels printed of a given color, thealgorithm is similar, however one would chose between the nearest 2levels of the total number of levels.

A revised preferred algorithm for multilevel printing that maintains theaverage color value AND the noise characteristics across the transitionregion is as follows:

-   -   A. For each pixel in each color plane that would have been used        in the single region printing process, determine the        corresponding Alpha l(x,F) and Alpha r(x,F) values, in the        transition region and    -   B. multiply Alpha l (x,F) times the corresponding color value        F (x) of the single region color plane, resulting in a target        local density for the left raster D l (x,F);    -   C. multiply Alpha r (x,F) times the corresponding color value        F (x) of the single region color plane, resulting in a target        local density for the right raster D r (x,F);    -   D. Halftone both the left and right rasters with the tri-level        halftoning process, each using uncorrelated random variables to        select among the three nearest levels.        Experimental Methods of Determining Alpha

Though there is a theoretical method for calculating Alpha, in mostcases, an empirical process is more reliable since it avoids makingassumptions about the relationship between dot density and imagedensity.

From the above analytical calculation of Alpha, it was observed that ifAlpha l can be obtained as a function of Alpha r and F, Alpha r andAlpha l can be determined as a function of position. Since Alpha makesits biggest changes at high values of F, the most experimental data wasgenerated in the region of high values of F. Refer here to FIG. 15 whichis discussed as a part of the procedure of the present inventiondiscussed below.

The procedure for producing Alpha reduces to:

-   -   A. Print patches 127 from the left raster in densities of        approximately F=0.1, 0.6, 0.8, 0.95 using the stochastic        halftoning method to be used later in printing the left and        right regions. These patches will be a reference to which pairs        of F·Alpha l and F·Alpha r are matched and chose Alpha l (F) and        Alpha r (F) pairs which accurately reproduce F when printed with        overlapping left and right rasters;    -   B. For each such patch, (which is the reference density) print        an array of patches with normal density F and Alpha l 126        varying from about 0.05 to about 0.5, and Alpha r 128 varying        from about 0.5 to 1.0 in steps of 0.05, and using different,        uncorrelated masks for the stochastic halftoning pattern for the        right and left printheads. The entries to be put in the table        below are color patches that simulate regions of the transition        zone.    -   C. For each reference density, pick the series of Alpha l and        Alpha r pairs that correspond, i.e., for each of the colors F        above, obtain pairs.

Reference Density Alpha l Alpha r 0.1 .05 Corresponding value 0.1Corresponding value 0.25 Corresponding value 0.5 Corresponding value 0.6.05 Corresponding value 0.1 Corresponding value 0.25 Corresponding value0.5 Corresponding value 0.8. 0.05 Corresponding value 0.1 Correspondingvalue 0.25 Corresponding value 0.5 Corresponding value 0.95 0.05Corresponding value 9.1 Corresponding value 0.25 Corresponding value 0.5Corresponding value

-   -   D. Build an interpolated function of Alpha r (F) vs. Alpha l        (F). An interpolation function simply interpolates between the        data points, and is part of standard mathematical calculation        packages such as Mathematica.    -   E. Compute Alpha l=Alpha l(x, F) and Alpha r=Alpha r(x,F) as        described above.

The table above is an example of generating a correspondence betweenAlphal(F) and Alphar(F). Where more accuracy is required, a greaternumber of data points need to be taken.

The above is thus a prescription of the present invention forreproducing uniform color across a transition region, using eitherexperimental or analytical approaches.

Multicolor Transitions

To complete the process of multicolor printing, the procedure ofmeasuring test patches illustrated in FIG. 15 and described in detailabove should be done for each of the colors used in the printer(typically CYMK, and sometimes CC′YMM′K or even CYMKOG or othervariants). The transition regions for each color can, and ideallyshould, be disjointed. Ideally, they would be adjacent to each other, atapproximately the same spacing as the color to color spacing on theprintheads so that the printheads do not have to travel any extradistance to implement the algorithms above, although this is not anabsolute requirement.

Flow Chart of the Present Invention for Design and Operation ofMulti-Printhead Printers

Reference here is made to FIG. 16 that is discussed in detail in thissection.

To design a printer for quality that is consistent with cost the presentinvention offers the following process:

-   -   A. Make dot placement accurate (130), and repeatable (ideally        within 1 or at least a few dot diameters):        -   a. First, identify corresponding nozzles on each printhead            (which may not be the same number in all printheads, because            of a paper axis offset due to manufacturing tolerances).            Renumber the nozzles that print the same horizontal dot row            to have the same number, and don't use nozzles that have no            counterparts on all the other printheads. This would            typically be done by printing a test pattern with the            cartridges and measuring the nozzle offsets in the paper            axis with an on-board optical sensor, or printing a test            pattern sensitive to nozzle misalignments, and having the            user select which pattern is least misaligned (132);        -   b. Second, identify any scan axis offsets between the            nominal location of a printhead, and the actual location,            and enter that offset in the print tables such that no dot            will be printed offset more than about 1 pixel spacing            either in the scan axis, or the paper axis. This may be done            through the use of optical sensors observing offsets of            alignment test patterns, or by having the user select which            pattern is nearest optimum, and entering that data into a            table via one of many possible means (134);    -   B. Implement the appropriate number of dots per pixel (136):        -   a. This should ideally be at least 2 dots of each color, and            possibly more than 1 dye load color;    -   C. Stabilize, and if necessary correct the colors from each        inkjet pen:        -   a. Use cartridges or printheads whose colors are matched,            and stable, and perceived to give identical colors. If not            matched off the production line, individually calibrate each            head to match, and/or build in printer controlled mechanisms            to adjust colors appropriately. This could include            occasionally calibrating the head, and adjusting some head            parameter such as temperature or bias voltages (138);    -   D. Define a number of print regions corresponding to the number        of printhead arrays (140):        -   a. Make the print regions of same width, and (optionally)            make the borders of the regions pass through easy transition            regions (142);        -   b. Use a transition region width at least 5 times, and            preferably 40 times wider than the greatest anticipated            residual corresponding raster offsets after calibrations            (144);    -   E. Measure the Alphas as described above for each color and for        each relevant print mode (draft, best, photo, etc.), and print        medium (plain paper, photopaper, etc.) (146);        -   a. Use a the test pattern described above and shown in FIG.            14 to measure Alpha l and Alpha r pairs for a series of            colors, and using the methodology illustrated, and discussed            in relation to, FIGS. 11A-E determine Alpha l (x, F), and            Alpha r (x, F). Use non-correlated pseudo random variables            for the masks for the right and left rasters when generating            the test patterns;    -   F. When the printer is an actual operation, print Alpha l·F on        the left raster, and Alpha r·F on the right raster for each        color plane, using uncorrelated mask random variables for the        left and right rasters to generate the actual pattern of drops        fired for all colors (148).

Depending on the cost objective of the printer, some of these steps canbe left out or modified, with a certain amount of penalty in terms ofvisible overlap artifacts. For example, in low cost printers, measuringand correcting the color value in the left and right regions may be donemanually (by the end user of the printer by comparing printed colors ofa left patch to alternative colors of a right patch, and entering thebest match via the computer to a table, which can then be used todevelop the corrections similar to what is currently done in some priorart printers to align the printheads). In higher cost printers, ascanning spectrophotometer or densitometer in the printer could be usedto measure and feedback the same data to the printer controller orinternal processor. In the lowest cost printer, no color calibrationwould be done by the user, and default color tables would be implementedin the printer. Those calibrations could be standard for all producedprinters, or calibrated just once on a production line for a specificprinter's mechanical tolerances.

Similarly, the methods for measuring and correcting for nozzle offsetscan be manual, automatic, or not at all, depending on the cost/qualitytrade-offs appropriate for the printer target application.

Printing algorithms of the present invention always use the dotreplacement algorithms discussed above, and could use default Alphafunctions, or Alphas that were manually or automatically readperiodically, or when an event occurs (e.g., the paper is changed).

Third Preferred Embodiment Black Only Printers with Further SpeedImprovements

The third embodiment of the present invention results in a black onlyprinter with a speed above 30 pages per minute using 1 inch swathprintheads (the current state of the art printhead swath width). Inbrief, 3 black printheads are mounted on a ganged array, as shown inFIGS. 17 and 18. More specifically, FIG. 17 shows a simplified top viewof a black only printer, with 3 black printheads (106KL, 106KM and106KR) scanning 3 separate corresponding regions or columns (152L, 152Mand 152R) and transition regions 150L and 150R of printable region 102of substrate 100 with the printheads aligned one with the other to eachcover approximately one third of a swath across the printable region 102substantially perpendicular to the columns.

FIG. 18 shows a simplified perspective view of a first alternative ofthe third embodiment of FIG. 17 with the printheads mounted in a rigidrelationship with respect to each other. The printheads are mounted incarriage 164 in a fixed relationship to each other with carriage 164affixed to belt 160 with belt 160 driven by sprocket 162 that is underthe control of the printer controller. For simplicity, other well knowndetails of the inkjet printer are not shown.

With the printheads in the configuration shown in FIGS. 17 and 18, ifthe print speed for a printer with a single black printhead is 21 pagesper minute, the corresponding print speed for a printer thatincorporates the 3 printhead configuration of the present invention willbe nearly 41 pages per minute at 2 g acceleration of the printheads. Theonly practical limit to the number of black printheads aligned to eachother as shown, other than the physical width of each printhead, is theacceleration and deceleration time, which should be kept well under theprinting time. For typical prior art office printers, horizontal scanspeeds are conveniently 30 inches per second. If the swath time is setequal to the sum of the deceleration time and the acceleration time,T_(swath)=T_(deceleration)+T_(acceleration), the minimum printableregion 102 width for 2 g acceleration when calculated is about 1.88inches, and about 1.2 inches for 3 g's which should be practical for ablack only printer. A printer with 5 black heads in a configurationsimilar to that of FIGS. 17 and 18 and 3 g acceleration would have printspeed of about 64 pages per minute.

FIG. 19 shows a close-up perspective view of the configuration of FIGS.17 and 18 with the addition of a printhead service station 170,including capping mechanisms 174 and wiping mechanism 176, in closeproximity to carriage 164 under the paper path, so that the gangedprintheads do not have to be moved over to the side of the printer(which is typically done in the prior art) which would result in a muchwider printer than prior art printers. Thus, the printer would not haveto be much wider than the paper being printed upon. Employing a servicestation 170 under the paper path is desirable in most cases wheremechanically ganged printhead arrays are used. The service station 170has a mechanism to move it downward and out of the way during printingand upward to cap the printheads when carriage 164 in the home positionby means of shaft 172 that is driven by a mechanism (not shown) that iscontrolled by the main printer controller. The actual carriage 164 andservice station 170 can be configured in a number of ways to accommodatedifferent physical printer configurations that might be adopted by themanufacturer.

In printers employing the 3 printhead embodiment of the presentinvention is not limited to having the printheads rigidly coupledtogether, however rigid coupling would be the simplest implementationfor this embodiment of the present invention.

Alternative Methods to Mount and Move the Printhead Arrays Include:

-   -   A. Rigidly attaching the arrays together with a linkage, or        common carriage 164 as in FIG. 19, thus saving additional motors        and additional motion feedback servo loops. In this situation,        it is highly desirable to have the capping and wiping function        performed by an apparatus under the paper path. Otherwise the        printer would have to be as wide as the width of the paper plus        the width of the printhead carriage. This method would result in        the lowest cost implementation of most printers that incorporate        this embodiment of the present invention.        -   a. Alternatively, the capping and wiping could be            accomplished by rotating the printheads upward through at            least 90° around the scan axis to a capping position above            the paper path.    -   B. FIG. 20 shows a simplified perspective view of a second        alternative of the third embodiment of the present invention        with the printheads (106KL, 106KM and 106KR) mounted        individually and connected to either the front or the back of        the drive belt 160 (shown here connected to the front of belt        160). Attaching multiple printheads or printhead arrays 178B to        common drive belt 160 would use one motor and would probably        require a position feedback servo loop (not shown) for each        array. The arrays could move in parallel motion, or (using the        other side of the belt for some of the individual printhead        arrays 178B) contrary motion. In the belt drive implementation,        it would be very desirable to have the capping and wiping        function performed by an apparatus under the paper path.        Otherwise the printer would have to be as wide as the width of        the paper plus the separation of the two outer most distant        printheads.        -   a. This method would have a slightly higher cost, however it            might result in less moving mass and vibration than in            case A. immediately above.    -   C. FIG. 21 shows a simplified perspective view of a third        alternative of the third embodiment printer of the present        invention, with the printheads (106K1, 106K2 and 106K3) mounted        individually on, and controlled individually by, separate drive        belts (182A, 182B and 182C), plus a novel paper drive mechanism        180. Each of drive belts (182A, 182B and 182C) are driven by        separate motors coupled to separate position feedback servo        loops (not shown). While in this case it is less advantageous to        have a capping and wiping function under the paper path, since        the printheads can be positioned adjacent to each other and thus        would not require as much space at the side of the printer as        when the printheads remain in a fixed relationship with respect        to each other as in FIG. 19, it is still desirable. If capping        and wiping are not under the paper path, the printer would have        to be at least the width of the paper 100 plus the width of the        all 3 printhead arrays (not shown). FIG. 21 also shows a pair of        ribbed drive rollers 180A and 180B with the drive wheels 181 on        each of the drive rollers offset from each other with drive        roller 180A above paper 100 and drive roller 180B below paper        100 which when driven by a drive system controlled by the        printer controller induce a corrugation pattern in paper 100        with each of the rollers above paper 100 creating a downward        indentation while the rollers below paper 100 are creating an        upward indentation. One such means includes inducing a low        amplitude (approximately 5 to 10 mil) ripple in the paper using        offset ribbed rollers 180 (with the wave crests like a        corrugated roof coming out of the printer). This induces a high        bending moment while the paper is under the printheads. The        corrugation gives paper 100 a three-dimensional shape to stiffen        paper 100 in the region under the printheads; this is        particularly important in two-sided printing.

To get the desired speed improvements over the prior art printers, theprintheads need only print on substantially disjoint regions of thepaper (e.g., as in the defined regions shown in FIG. 17); how the headsare actually positioned is a secondary factor that affects issues suchas cost, printer size, and vibration.

FIG. 26 is a simplified printer implementation that includes a combinedblock diagram of a print process and control subsystem together with theprimary mechanical printer elements (similar to those shown in FIG. 20)of a paper path. Process and control subsystem 240 includesmicroprocessor 244 that coordinates the operation of the variousfunctions of the printer. In communication with microprocessor 244 ismemory 242 that contains various print algorithms, data and data look-uptables to perform the conventional and present invention functions ofthe printer. Alternately, or in combination with, the information storedin memory 242, there may be additional print algorithms stored in thehost computer (not shown) which are used to process the data to beprinted from various programs that are being used on the host computerwith that raw and/or processed data being transferred to microprocessor244. Then the data is further processed by microprocessor 244 andtransferred to paper control 246, firing control 248 and carriagecontrol 250 via busses 262, 264, and 266, respectively, in the neededsequence as determined by the printer operational algorithm(s). In theembodiment shown here, paper control 246, via buss 256, activates motor270 which in turn is coupled to the paper drive mechanism that is shownhere consisting of drive rollers 180A and 180B between which the paper(not shown) passes and is advanced and retracted thereby as determinedby the print algorithm then in use. Firing control 248, via buss 254, iscoupled to each of printhead arrays 106KL, 106 KM and 106KR, to causeeach printhead to individually expel drops of ink at precise points intime onto the paper as determined by the same print algorithm thatdetermines the positioning of the paper by paper control 246. Similarly,carriage control 250, via buss 252, is coupled to motor 272 that drivesbelt 160 to laterally position the printheads laterally at the rightpoint in time with each of the printheads, or a carriage 178B on whichthey are each mounted, being attached to belt 160. Thus it can be seenthat with microprocessor 244 coordinating the operation of each ofpaper, firing and carriage controls, the paper and printhead carriageare positioned at the correct location in time that the printheads arefired to deliver drops of ink to the correct location on the paper.

Fourth Preferred Embodiment Wide Format Printing

Multiple printhead arrays (i.e., 2 or more arrays) would be especiallyuseful in wide format printing, where it is uneconomical and unfeasibleto make much larger swath printheads, and definitely unfeasible to makepage width printhead arrays. In the large format case, many more than 2printhead arrays would be useful. To make an effective multiple headprinter that is not extraordinarily wide, and have a simple controlmechanism, the printer would be a cut sheet printer (where the printheadassemblies could be serviced after each sheet), and have the servicestation under the paper path.

An alternative implementation of the service station could be above thepaper path, with the printheads rotated to mate with the servicestation.

Alternatively, such a wide format printer could use option C of thethird preferred embodiment above with separate servo motors and positionsensors.

Other Considerations for the Third and Fourth Embodiments

It is not necessary that the printhead arrays all move in unison,although that might, in most cases, be the simplest design. For example,other designs are possible where the heads are coordinated in “contrarymotion” that could result in less printer vibration. Alternatively,printheads that move in a fashion that is almost 180 degrees out ofphase, but not quite, from each other would allow much less vibration,and the adjacent printheads would not run into each other. For example,a printer with a three printhead assembly as described in the thirdpreferred embodiment could have the printheads moving with 120 degreephase differences as one proceeds from right to left. This would resultin virtually no vibration.

Conceivably, each printhead array, and even each printhead alone, couldhave its own motion control subsystem, coordinated in a fashion tooptimize print speed and quality, while at a minimum system cost.Separate motion control systems would also provide better flexibilityfor dealing with different width substrates to maximize throughput thanin systems where printhead spacing is fixed.

The benefits of the above implementations are much higher speed atlittle additional cost, and the ability to use existing printheaddesigns to make much faster printers.

Fifth Preferred Embodiment Double Sided Printing

Most printers are inherently incapable of printing simultaneously onboth sides of the sheet at the same time. Laser printers have a drum toprovide a pattern of toner that is applied to one side of the paper by aroller pressing the paper onto the drum from the opposite side of thepaper. Offset presses apply inked material on one side of a piece ofpaper while applying pressure via a platen on the other side of thepaper. Thus, double sided printing has, to date, required sequentialprinting first on one side, then on the other, resulting in large,expensive machines. Alternatively, for inkjet printers that can onlyprint on one side of a sheet at a time to be capable of printing on bothsides of the same sheet have to be able to, in effect, turn the sheetover, long side for long side for printing on the second side afterprinting of the first side has been accomplished and the ink has driedsufficiently before passing the sheet through the printer a second time.The “turning of the sheet over” to print the second side could beaccomplished by first drawing the sheet back into the printer and asdoing so using a “reverser” which directs the narrow end of the sheetthat was the bottom of the page on the first pass through 180° and feedit into the print mechanism first for printing on the second side.However, the end that is now being feed back into the print mechanismwas the bottom edge of the sheet when printed on the first side and isnow the top edge of the sheet thus if printing proceeds as if theorientation of the sheet had not changed, the printing of the secondside of the sheet will be upside down from that on the first printedside unless an additional function is performed before the second sideis printed. To have the top of each printed side be adjacent the sameedge of the sheet, what can be done is it to electronically invert theprinting order for the second side so that printing proceeds by printingthe text or image upside down relative to the image on the first side,i.e., from the bottom of the image to the top of the image. Stated inanother way, when the first side of the sheet is printed, the top of theimage first exits the print mechanism, while when the second side of the“reversed” sheet is printed, the bottom of the image on the first sideof the sheet exits the print mechanism first.

Both of the prior art methods for printing on both sides are inherentlymore expensive, more complicated, and in the case that uses a“reverser”, limit throughput because the reverser has to wait for thefirst side of a sheet to be completely printed and dried, beforereversing the paper, thus forcing the print mechanism to idle while thepaper is reversed.

Another benefit of the present invention is that a sheet of paper can beprinted with scanning inkjet printheads simultaneously on both sides ofthe sheet with dedicated printhead arrays. Further, multiple printheadarrays can be utilized on both sides of the sheet to further increasethroughput. This can be done on cut sheet as well as web based printingformats.

FIG. 22 shows a simplified perspective view of a fifth preferredembodiment of the present invention to permit black printing incorresponding regions on both sides of the paper. FIG. 22 shows two setsof ganged black printheads (in this view 106KT1, 106KT2 and 106KT3 areabove page 100; and 106KB1, 106KB2 and 106KB3 are below page 100), oneon each side of paper 100. This is an extension of the third embodimentshown in FIG. 17. With that black printhead array configuration of 3printheads adjacent each side of page 100, the printer could achieve afull page print rate of 41 double sided sheets or 82 pages per minute,at a relatively inexpensive printer manufacturing cost compared to priorart technologies; with black arrays of 5 printheads, one could achieve64 double sided sheets or 128 pages per minute.

Turning now to FIG. 23 there is shown a simplified perspective view of afifth preferred embodiment of the present invention configured to permitblack and color printing on both sides of the sheet 100 with double theblack print speed over the first preferred embodiment, shown in FIGS. 4Aand 4B.

FIG. 23 shows a single scanning color printhead array on each side ofsheet 100, with black printheads on each end of the array (106KT, 106MT,106YT, 106CT and 106K′T above sheet 100; and 106K′B, 106CB, 106YB, 106MBand 106KB below sheet 100. With such an arrangement, it would bepossible to obtain a black only, double sided print rate of 35 doublesided sheets, or 70 pages per minute (ppm). Color print speed for theconfiguration of FIG. 23 depends primarily the interlace factor and theovertravel of the printhead arrays, yet this configuration could stillbe 4-15 times faster than existing prior art inkjet modes of doublesided color printing.

Printing with multiple color printhead arrays on both sides of sheet100, similar to the black only embodiments of the present invention asshown in FIG. 22, for example, is also a variation of the presentinvention. Such a configuration, with 3 color printhead arrays on eachside of sheet 100, each with 1 inch swath and 1 inch overtravel plus aninterleave factor of 2, the print speed achievable is about 11 doublesided full color sheets or 22 pages per minute.

In the case of double sided printing one cannot support the paper frombelow in the print zone, as is done in the prior art. Therefore, it maybe useful to provide stiffening means (e.g. rollers 180A and 180B as inFIG. 21) to keep the paper exactly half way between the printheads, andthereby preventing the printheads from hitting the paper and smearingthe ink. One such means includes inducing a low amplitude (approximately5 to 10 mil) ripple in the paper using offset ribbed rollers 180 (withthe wave crests like a corrugated roof coming out of the printer). Thisinduces a high bending moment while the paper is under the printheads.Alternatively, the printer may support the paper at the exit of theprint zone on an air pillow impinging on both the top and the bottomsides of sheet 100, and to prevent cockle in the print zone by ensuringthat wet paper stays in the print zone less than 5 seconds—which iseasily possible with the higher speed implementations made possible byusing multiple printheads configurations of the present invention.

Sixth Preferred Embodiment Segmented Page Width Arrays

Ideally, page width arrays printing one or more colors could beconstructed as a single monolithic structure with all nozzles evenlyspaced apart across the page. However, generally this is noteconomically feasible because of low fabrication yields, or theinability to source substrates of the required size and properties.Hence, many prior art pagewidth arrays are constructed of printheadsegments arranged in a fixture in a regular pattern to approximate amonolithic design as shown in FIGS. 24A and 24B. In these figures, whichare wire frame depictions to make visible the relationships if thevarious elements, the array substrate 194 supports print segments 196(on the bottom side of the support, and between the paper and thesupport) that are rotated at an angle to ensure that the dots fromindividual nozzles 198 overlap. However, alignment of the segments isnot perfect, and therefore nozzle spacing irregularity occurs at theadjoining regions 197, which can result in vertical lines or gaps in theprinted image on the page. Errors as small as 1/10 of a pixel dimensioncould be visible—which at 600 dpi, is about 0.16 mils, or about 4microns.

Thus, in the present invention printing segments 196 are overlapped(199) slightly, as shown in FIGS. 24A and 24B, and the present inventionuses the same algorithm for this application as discussed above forcomputing the dot densities to be printed by the rasters (in this case,each part of the print line printed by the left and right segment).Specifically, the overlapping region 199 is analogous to the transitionzone 116 of FIGS. 4A and 4B. Each line printed with each adjacentprinthead segment 196 is printed with a halftone derived fromuncorrelated masks, and the densities F in the “transition region”(overlapping region) are multiplied by Alpha l for the left printheadsegment and Alpha r for the right print head segment, and printed withthe halftone mask for that printhead segment. Thus, for example, ifsegments 196 are 1 inch wide at 600 dpi with potentially 1 dot placementerror, segments 196 would be over lapped by 0.067 inches on each end.

Seventh Preferred Embodiment Line Feed Errors

The difficulty of accurately advancing paper in a printer is a wellrecognized problem in the industry. Line feed errors are visible aslight or dark horizontal bands on the paper that are particularlyvisible in uniform color regions. Due to the recognition of thisproblem, most printers incorporate interlace printing, and advance thesheet only a fraction of the swath width (usually a third or a fourth aswath width) when printing in color, as shown in prior art FIG. 25A.This interlacing and corresponding fractional swath advance has twoeffects: smaller advance errors are produced since the paper is moving asmaller distance (the result of fractional swath advance), therefore thewidth of each white band is smaller, and the color intensity of eachband that is smaller by ⅓ or ¼ of the average intensity because eachregion where a band might appear is also printed in other passes in themiddle of the printhead (the result of shingling). While this techniqueis fairly effective, it comes at a cost of 3 to 4 times the number ofpasses across the paper than would otherwise have been necessary hadthere been no paper advancement error, thus slowing the printing a likeamount.

Prior art FIG. 25A shows as an example of that printing technique withthe printed area from a 1 inch swath printhead employing 3 wayinterleave, and therefore advancing only 0.33 inches per swath. Thevertical cross hatched region (strips 204, 206 and 207) represents thearea printed in the first swath (though only ⅓^(rd) of the print nozzlesare fired at any one time); the right leaning cross hatched region(strips 206, 207 and 208) represents the area printed by the secondswath; and the left leaning cross hatched region (strips 207, 208 and209) represents the area printed in the third swath. At the end of 3scans, only ⅓^(rd) of an inch has been fully printed, namely region 210(strip 207).

The present invention provides a solution to the print slow down penaltyof the prior art discussed above with respect to FIG. 25A. The presentinvention approach is illustrated with the aid of FIG. 25B, with penaltybeing avoided by using the feathering scheme of the present invention.Specifically, the overlap regions 214 and 218 (similar to verticaltransition region 116 of FIG. 4A) are feathered by: printing each swathusing uncorrelated mask halftones, and in the overlap regions 214,multiplying the desired density F by Alpha t (for the top band 212) andAlpha b (for the bottom band 220). Alpha t is the same function of y asAlpha l was of x; Alpha b is the same function of y as Alpha r is of x).The width of the transition/overlap region would be 40 times the maximumanticipated line feed error. In FIG. 25B, the first swath 212(vertically cross hatched) is completed; and the second swath 216 (rightleaning diagonal cross hatched) overlaps the first swath in overlapregion 214. Thus, for a 1 inch high swath, the overlap would typicallybe 0.067 inches—or 6% of the swath height. Thus, in effect, the printspeed would be increased almost by the interlace factor—reducing theinterlace factor in this embodiment to nearly 1. Similarly, the thirdswath 220 (left leaning diagonal cross hatched) overlaps the secondswath in overlap region 218.

Eighth Preferred Embodiment Color Bidirectional Printing

Color printing in the prior art is usually done only unidirectionallybecause:

-   -   A. The ink droplets that are offset in one direction when        scanning to the right are offset in the opposite direction a        varying amount when scanning to the left. This results in a        pattern of shifting colors when overlaying rasters are shifted        slightly, known as moiré patterns in the prior art.    -   B. When one ink color X is deposited on top of a previously        deposited color Y, the resultant printed color is different than        if color Y is deposited on previously deposited color X.

Bi-directional scans in the present invention can avoid these prior arteffects by:

-   -   A. Using independent, uncorrelated masks for each of the colors,        in each direction. Therefore, when an offset because of        mechanical imperfections occurs, some of the drops of different        colors that now overlap more than in a mechanically perfect        printer are compensated for by drops that are overlapped less,        due to the effect of the use of uncorrelated masks—thus making        the perceived color unaffected by mechanical offsets or drops of        ink.    -   B. The intrinsic difference in color that occurs when inks are        deposited in different orders can now be measured and reversed        in the printer color tables, which would now be different for        each direction of printing.

Thus, by using independent masks for each color in each direction ofprinting, and by developing different color tables for each direction ofprinting, bidirectional color printing is enabled, speeding up colorprinting by another factor of about 2.

Combinations of Embodiments

While the various embodiments have been discussed separately, acombination of various of the embodiments of the present invention wouldbe useful in the same printer.

It is generally possible to combine the various embodiment methods ofthe present invention discussed above. For example, it is possible to,for color printing:

-   -   A. Use multiple printheads, scanning corresponding regions on        one side of the paper for a 3× improvement (second and third        embodiments);    -   B. Use on both sides (2× improvement) (fifth embodiment);    -   C. Reduce interlacing by a factor of 3 to 8 (3 to 8 times speed        up) (seventh embodiment);    -   D. Implement bidirectional printing (up to 2× improvement)        (eighth embodiment);    -   E. In doing simultaneous printing on both sides, avoid the use        of paper reversers (large speed improvement for double sided        printing compared to the prior art) (fifth embodiment).

CONCLUSIONS, RAMIFICATIONS, AND SCOPE OF THE INVENTION

A method and apparatus of significantly increasing print speed at lowcost has been described. The means also enables double sided printing atreasonable costs and high throughput. Included is a method to buttadjacent regions without print artifacts.

The present invention increases the speed of ink jet printers either byapplying multiple print heads to print various regions simultaneously,or using the feathering algorithm presented that enables improvedefficiency.

The various embodiments of the present invention discussed above hasdemonstrated that a 6× improvement in black print rate, (even notaccounting for the fact that the method of the first embodiment doesn'trequire a reverser) is possible. For color, it has been demonstratedthat up to a 96× speed improvement is possible—with existing technology.

The methodology of the present invention also enables high qualityprinting with segmented pagewidth printheads, which is intrinsicallymuch lower cost than monolithic pagewidth printheads.

While several different embodiments and variations thereof have beendiscussed above, that is not intended to be a complete list of ways thatthe present invention can be accomplished, thus the present invention isnot to be limited to only those. The present invention and itsequivalents are all part of the present invention. One skilled in theart will recognize that the same results provided by the disclosedembodiments of the present invention could be achieved with differentconfigurations. Therefore the present invention is only to be limited bythe scope of the claims and their equivalents.

What is claimed is:
 1. An apparatus for printing on a print medium assaid print medium advances along a selected path, said apparatuscomprising: a print material delivery assembly including at least oneprint device with each print device having at least one nozzle, saidprint material delivery assembly disposed to deposit droplets of a printmaterial on at least a portion of said print medium a plurality oftimes; and a print firing controller to generate a print firing signalfor each, and coupled to, said at least one nozzle to selectivelydeposit said droplets onto said print medium utilizing correctionfactors to modify the number of droplets to be fired from acorresponding printhead at least one nozzle to achieve the desireddensity of said droplets using substantially uncorrelated halftone masksto independently adjust each print firing signal as each droplet isdeposited on a same nominal position of said print medium, the halftonemasks being substantially uncorrelated so that the halftone masks aredifferent from each other.
 2. The apparatus for printing on a printmedium as in claim 1 wherein said print material delivery assemblyincludes a printhead mechanism wherein said at least one print devicethat is at least one printhead with each printhead comprising aplurality of nozzles.
 3. The apparatus for printing on a print medium asin claim 2 wherein: said print material delivery assembly furtherincludes a transport system to retain and repeatedly bidirectionallyscan said printhead mechanism in a printhead transport path across saidselected path; and said print firing controller generates said printfiring signals to cause said at least one printhead to print droplets onsaid print medium as said printhead mechanism passes along saidprinthead transport path in both directions.
 4. The apparatus forprinting on a print medium as in claim 3 wherein said printheadmechanism includes at least two printheads each disposed to depositdroplets of the same material with each of said at least two printheadsspaced apart from each other as they are scanned along said printheadtransport path.
 5. The apparatus for printing on a print medium as inclaim 4 wherein said transport system includes a drive mechanism towhich each of said at least two printheads are coupled a fixed distancefrom each other when scanned in both directions.
 6. The apparatus forprinting on a print medium as in claim 4 wherein: said at least twoprintheads includes a first plurality of printheads; said transportsystem includes a second plurality of drive mechanisms, said firstplurality being equal to or greater than said second plurality, witheach of said drive mechanisms having coupled thereto at least oneprinthead; and said transport system controls said second plurality ofdrive mechanisms to maintain the printheads coupled to each of saiddrive mechanisms spaced apart from all other printheads when scanned inboth directions along said printhead transport path.
 7. The apparatusfor printing on a print medium as in claim 4 wherein said transportsystem includes at least two separately controlled drive mechanisms witheach drive mechanism having at least one printhead coupled thereto. 8.The apparatus for printing on a print medium as in claim 7 furthercomprises a system processor to determine the operational positioning ofeach printhead and instructs said transport system to operate each drivemechanism to position said printheads accordingly at each point in timewithout any interference one printhead with another when scanned acrosssaid print medium wherein the width of each of said printable regionsand shared transition regions results from the operational positioningof the printheads as determined by the system processor.
 9. Theapparatus for printing on a print medium as in claim 8 wherein theresulting width of each printable region and transition region can bedifferent from other printable regions and transition regions.
 10. Theapparatus for printing on a print medium as in claim 8 wherein eachprintable region has substantially the same width as each otherprintable region, and each transition region has substantially the samewidth as each other transition region.
 11. The apparatus for printing ona print medium as in claim 8 wherein the resulting spacing betweenprintheads varies at different points in time during operation of saidapparatus.
 12. The apparatus for printing on a print medium as in claim8: wherein said system processor instructs said transport system toadvance said drive mechanisms to move each printhead to a home positionin close proximity to each other outside said print medium selected pathwhen said apparatus in not printing; said apparatus further includes aprinthead service station located in said home position having both acapping mechanism and a wiping mechanism for each printhead; and whereinsaid system processor controls said printhead service station to cap andwipe said printheads when printing is not being performed.
 13. Theapparatus for printing on a print medium as in claim 2 wherein: saidprint material delivery assembly further includes a transport system toretain and repeatedly bidirectionally scan said printhead mechanism in aprinthead transport path across said selected path; said printheadmechanism includes at least one printhead each with at least one nozzledisposed to deposit droplets of said material having a first color andat least one other nozzle disposed to deposit droplets of said materialhaving a second color; said print firing controller generates said printfiring signals to cause each of said nozzles to print droplets on saidprint medium as said printhead mechanism passes along said printheadtransport path utilizing a first set of substantially uncorrelatedhalftone masks when said printhead mechanism is scanned in a forwarddirection followed by a reverse direction, and a second set ofsubstantially uncorrelated halftone masks when said printhead mechanismis scanned in a reverse direction followed by a forward direction; anddifferent color tables may be used for regions scanned in the forwarddirection followed by the reverse direction than the color tables usedfor regions that are scanned in the reverse direction followed by theforward direction.
 14. The apparatus for printing on a print medium asin claim 2 wherein: said printhead mechanism includes at least twoprintheads disposed to deposit droplets of the same material; said printmaterial delivery assembly further includes a transport system to retaineach of said printheads spaced apart from adjacent printheads and toscan said printheads in a printhead transport path along an x axis eachacross a corresponding printable portion of said print medium with anoverlapping transition region that is included in each adjacentprintable portion; and said print firing controller generates said printfiring signals for said printhead nozzles in adjacent printheads wheneach of said adjacent printheads is printing in said transition regionshared by the adjacent printheads utilizing said substantiallyuncorrelated halftone masks.
 15. The apparatus for printing on a printmedium as in claim 14 disposed to deposit droplets of a selected numberof materials on said print medium wherein at least one nozzle in each ofsaid at least two printheads in said printhead assembly is disposed todeposit droplets of each of said selected number of materials on said atleast a portion of said print medium utilizing said correction factorsand said substantially uncorrelated masks.
 16. The apparatus forprinting on a print medium as in claim 14 wherein said print firingcontroller is disposed to generate print firing signals to causeindividual printhead nozzles to deposit multiple droplets of saidmaterial to a same location on said print medium.
 17. The apparatus forprinting on a print medium as in claim 16 wherein said printhead nozzlesdeposit multiple droplets of said material during at least two scansacross a same path of said print medium.
 18. The apparatus for printingon a print medium as in claim 14 wherein the relationship between theprint firing signals for said printhead nozzles of each of adjacentprintheads in the corresponding transition region at the x axis positionwithin the transition region with one printhead to the left of the otherprinthead, and one printhead being to the right of the other printheadisF=F·Alpha l+F·Alpha r·(1−F·Alpha l) where: F=the print density of darkpixels desired to be delivered in the transition region; Alpha l=thefraction of said print density to be delivered from the left printhead;and Alpha r=the fraction of said print density to be delivered from theright printhead; with Alpha l gradually varying from 1 to 0 and Alpha rgradually varying from 0 to 1 with increasing x in the transitionregion, wherein the Alpha values represent the fraction of the value ofprint firing signal that would be applied to the corresponding printheadnozzles if only the left or right printhead were printing in thetransition region.
 19. The apparatus for printing on a print medium asin claim 18 wherein said correction factors are developed using afeathering algorithm, the feathering algorithm being calculated by Alphal and Alpha r.
 20. The apparatus for printing on a print medium as inclaim 14 wherein the relationship between the print firing signals forsaid printhead nozzles of each of adjacent printheads in thecorresponding transition region at any x axis position within thetransition region with one printhead to the left of the other printhead,and one printhead being to the right of the other printhead isD(F)=D(Alpha l·F)+{1−D(Alpha l·F)}·(D(Alpha r·F) where: D(F)=the imagedensity; F=the dot density in the transition region or, equivalently,the number of pixels dark per unit area; Alpha l=the fraction of pixelsused from the left printhead or, equivalently, the correction factor ofthe left printhead to set the fraction of F to be delivered by the leftprint head; and Alpha r=the fraction of pixels used from the rightprinthead or, equivalently, the correction factor of the right printheadto set the fraction of F to be delivered by the right print head; withAlpha l gradually varying from 1 to 0 and Alpha r gradually varying from0 to 1 with increasing x in the transition region, wherein the Alphavalue represents the fraction of the value of print firing signal thatwould be applied to the corresponding printhead nozzles if only the leftor right printhead were printing in the transition region.
 21. Theapparatus for printing on a print medium as in claim 20 wherein saidcorrection factors are developed using a feathering algorithm, thefeathering algorithm being calculated by Alpha l and Alpha r.
 22. Theapparatus for printing on a print medium as in claim 14 wherein a sum ofsaid print droplets deposited in said transition region to form an imageof a selected density by adjacent printheads is greater than the sum ofsaid print droplets printed by one printhead in a portion of an adjacentprintable region of a width equal to a width of said transition regionwhen printing said image of said selected density so the apparent printdroplet densities printed by said adjacent printheads are equalized witheach other when printing.
 23. The apparatus for printing on a printmedium as in claim 14, wherein said correction factors adjust the printdroplet densities to be deposited by the respective printheads.
 24. Theapparatus for printing on a print medium as in claim 14 wherein saidcorrection factors are generated with a correction table to generate theprint firing signals for use in adjacent printable portions and thetransition region.
 25. The apparatus for printing on a print medium asin claim 14 wherein said correction factors are generated withinterpolation functions of Alpha r (F) vs. Alpha l (F) to generate theprint firing signals for use in adjacent printable portions and thetransition region.
 26. The apparatus for printing on a print medium asin claim 14 further comprising: a print medium advancing mechanism toselectively position said print medium in said selected path; and asystem processor coupled to said print medium advancing mechanism, saidprint firing controller and said transport system to coordinate thepositioning of said print medium, and the firing and positioning of saidprintheads to selectively deposit droplets at desired locations on theprint medium.
 27. The apparatus for printing on a print medium as inclaim 26 wherein said system processor controls said advancingmechanism, print firing controller and said transport system to causeeach printable portion that corresponds to each said printhead on saidprint medium oriented parallel to said selected path with saidtransition region shared by adjacent printable portions also beingoriented parallel to said selected path.
 28. The apparatus for printingon a print medium as in claim 27 wherein there is at least one printheadhaving a same color print material corresponding to each adjacentprintable region and shared transition region.
 29. The apparatus forprinting on a print medium as in claim 2 wherein said transport systemincludes a single drive mechanism to which each of said at least twoprintheads are coupled a fixed distance from each other resulting in thewidth of each of said printable regions having a width that issubstantially equal to the spacing between two adjacent printheads thateach print to the same transition region.
 30. The apparatus for printingon a print medium as in claim 29 wherein said fixed distance issubstantially equal between each two adjacent printheads resulting insaid adjacent printable regions each having the same width.
 31. Theapparatus for printing on a print medium as in claim 1, wherein saidprint material delivery assembly includes: a printhead service stationabove or below said selected path of said print medium comprising: acapping mechanism for each of said at least one print device; and awiping mechanism for each of said at least one print device; and whereinsaid print material delivery assembly when fully assembled is notsubstantially wider than said selected path of said print medium. 32.The apparatus for printing on a print medium as in claim 31 wherein saidservice station is below a gap in said selected path of said printmedium when said at least one print device is depositing droplets onsaid print medium.
 33. The apparatus for printing on a print medium asin claim 32 further includes a service station elevation mechanism toraise said service station through said gap to cap or wipe said at leastone print device when printing is not being performed and to lower saidservice station to a rest position below said gap when printing is beingperformed.
 34. The apparatus for printing on a print medium as in claim1 wherein: said at least one print device comprises a print array havinga main body with a plurality of printing segments mounted thereon with aportion of adjacent ends of each pair of said printing segmentsoverlapping a corresponding same portion of said selected path with saidprint array mounted across said selected path with said printingsegments facing said selected path with each of said printing segmentshaving a preset number of print nozzles disposed to deposit droplets ofsaid print material on said print medium with the print nozzles thatoverlap said corresponding same portion of said selected path disposedto deposit droplets of said print material to a same region of saidprint medium; and said print firing controller generates said printfiring signal for, and is coupled to, each of said nozzles in eachprinting segment of said print array where, in the overlapped regionsutilizing said correction factors and feathering functions to achievethe desired density of said droplets using said substantiallyuncorrelated halftone masks to independently adjust each print firingsignal.
 35. The apparatus for printing on a print medium as in claim 1wherein: said print medium has a top side and a bottom side; said printmaterial delivery assembly includes: a first printhead mechanismdisposed to deposit droplets of said print material to said top side ofsaid print medium; and a second printhead mechanism disposed to depositdroplets of said print material to said bottom side of said printmedium.
 36. The apparatus for printing on a print medium as in claim 35:wherein said first and second printhead mechanisms each includes atleast two printheads disposed to deposit droplets of the same material;said print material delivery assembly further includes a transportsystem to retain each of said first and second printhead mechanisms withspaced apart adjacent printheads in each of said first and secondprinthead mechanisms and to scan said printheads in a printheadtransport path each across a corresponding printable portion of saidprint medium with an overlapping transition region that is included ineach adjacent printable portion; and wherein said print firingcontroller generates said print firing signals for said printheadnozzles in adjacent printheads in each of said first and secondprinthead mechanisms when each of said adjacent printheads is printingin said transition region shared by the adjacent printheads utilizingsaid substantially uncorrelated halftone masks.
 37. The apparatus forprinting on a print medium as in claim 36 wherein said transport systemincludes one drive mechanism to which each of said first and secondprinthead mechanisms are coupled with adjacent printheads a fixeddistance from each other when scanned in both directions.
 38. Theapparatus for printing on a print medium as in claim 36 wherein saidtransport system includes a first drive mechanism to scan said firstprinthead mechanism in said printhead transport path across saidselected path with adjacent printheads spaced apart from each other anda second drive mechanism to scan said second printhead mechanism in saidprinthead transport path across said selected path with adjacentprintheads spaced apart from each other.
 39. The apparatus for printingon a print medium as in claim 2: wherein said print material deliveryassembly further includes a transport system to retain and repeatedlyscan said printhead mechanism in a printhead transport path across saidselected path; wherein said print firing controller generates said printfiring signals to cause said at least one printhead nozzle to print aswath of droplets on said print medium with said swath having a presetswath height as said printhead assembly passes along said printheadtransport path; a print medium advancing mechanism to selectivelyadvance said print medium in steps in said selected path with each stephaving a length that is shorter than said swath height resulting in eachsuccessive swath overlapping an immediately preceding swath in atransition region; wherein said print firing controller uses afeathering function when printing in said transition region.
 40. Theapparatus for printing on a print medium as in claim 34 furthercomprising: a print medium advancing mechanism to advance said printmedium in discrete steps beneath said print array; and a systemprocessor coupled to said print medium advancing mechanism and saidprint firing controller to coordinate the positioning of said printmedium, and the firing of said nozzles in each said printing segment toselectively deposit droplets of said material in successive swathsacross said print medium.